Keep argv[0], add COSMOPOLITAN_PROGRAM_EXECUTABLE (#980)

* Introduce env.com

Handy tool for debugging environment issues.

* Inject path as COSMOPOLITAN_PROGRAM_EXECUTABLE

`argv[0]` was previously being used as a communication channel between
the loader and the binary, giving the binary its full path for use e.g.
in `GetProgramExecutableName`. But `argv[0]` is not a good channel for
this; much of what made 2a3813c6 so gross is due to that.

This change fixes the issue by preserving `argv[0]` and establishing a
new communication channel: `COSMOPOLITAN_PROGRAM_EXECUTABLE`.

The M1 loader will always set this as the first variable. Linux should
soon follow. On the other side, `GetProgramExecutableName` checks that
variable first. If it sees it, it trusts it as-is.

A lot of the churn in `ape/ape-m1.c` in this change is actually backing
out hacks introduced in 2a3813c6; the best comparison is:

    git diff 2a3813c6^..

test/libc/calls/getprogramexecutablename_test.c

@@ -0,0 +1,104 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/dce.h"
+#include "libc/limits.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/testlib/subprocess.h"
+#include "libc/testlib/testlib.h"
+
+static char *self;
+static bool skipcosmotests;
+
+void SetUp(void) {
+  self = GetProgramExecutableName();
+}
+
+void SetUpOnce(void) {
+  if (!getenv("COSMOPOLITAN_PROGRAM_EXECUTABLE")) {
+    fprintf(stderr,
+            "warning: old ape loader detected; skipping some tests %m\n");
+    skipcosmotests = true;
+  }
+  testlib_enable_tmp_setup_teardown();
+}
+
+__attribute__((__constructor__)) static void Child(int argc, char *argv[]) {
+  if (argc >= 2 && !strcmp(argv[1], "Child")) {
+    ASSERT_EQ(3, argc);
+    EXPECT_STREQ(argv[2], GetProgramExecutableName());
+    exit(0);
+  }
+}
+
+TEST(GetProgramExecutableName, ofThisFile) {
+  EXPECT_EQ('/', *self);
+  EXPECT_TRUE(
+    endswith(self, "test/libc/calls/getprogramexecutablename_test.com"));
+}
+
+TEST(GetProgramExecutableName, nullEnv) {
+  SPAWN(fork);
+  execve(self, (char *[]){self, "Child", self, 0}, (char *[]){ 0 });
+  EXITS(0);
+}
+
+TEST(GetProramExecutableName, weirdArgv0NullEnv) {
+  SPAWN(fork);
+  execve(self, (char *[]){"hello", "Child", self, 0}, (char *[]){ 0 });
+  EXITS(0);
+}
+
+TEST(GetProgramExecutableName, weirdArgv0CosmoVar) {
+  if (skipcosmotests) return;
+  char buf[32 + PATH_MAX];
+  stpcpy(stpcpy(buf, "COSMOPOLITAN_PROGRAM_EXECUTABLE="), self);
+  SPAWN(fork);
+  execve(self, (char *[]){"hello", "Child", self, 0}, (char *[]){ buf, 0});
+  EXITS(0);
+}
+
+TEST(GetProgramExecutableName, weirdArgv0WrongCosmoVar) {
+  if (skipcosmotests) return;
+  char *bad = "COSMOPOLITAN_PROGRAM_EXECUTABLE=hi";
+  SPAWN(fork);
+  execve(self, (char *[]){"hello", "Child", self, 0}, (char *[]){ bad, 0});
+  EXITS(0);
+}
+
+TEST(GetProgramExecutableName, MovedSelf) {
+  char buf[BUFSIZ];
+  ASSERT_SYS(0, 3, open(GetProgramExecutableName(), O_RDONLY));
+  ASSERT_SYS(0, 4, creat("test", 0755));
+  ssize_t rc;
+  while ((rc = read(3, buf, BUFSIZ)) > 0) {
+    ASSERT_SYS(0, rc, write(4, buf, rc));
+  }
+  ASSERT_EQ(0, rc);
+  ASSERT_SYS(0, 0, close(4));
+  ASSERT_SYS(0, 0, close(3));
+  ASSERT_NE(NULL, getcwd(buf, BUFSIZ - 5));
+  stpcpy(buf + strlen(buf), "/test");
+  SPAWN(fork);
+  execve(buf, (char *[]){"hello", "Child", buf, 0}, (char *[]){ 0 });
+  EXITS(0);
+}