Rewrite getcwd()

This change addresses a bug that was reported in #923 where bash on
Windows behaved strangely. It turned out that our weak linking of
malloc() caused bash's configure script to favor its own getcwd()
function, which is implemented in the most astonishing way, using
opendir() and readdir() to recursively construct the current path.

This change moves getcwd() into LIBC_STDIO so it can strongly link
malloc(). A new __getcwd() function is now introduced, so all the
low-level runtime services can still use the actual system call. It
provides the Linux Kernel API convention across platforms, and is
overall a higher-quality implementation than what we had before.

In the future, we should probably take a closer look into why bash's
getcwd() polyfill wasn't working as intended on Windows, since there
might be a potential opportunity there to improve our readdir() too.

libc/stdio/get_current_dir_name.c

@@ -0,0 +1,45 @@
+/*-*- 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 2020 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/calls/struct/stat.h"
+#include "libc/mem/mem.h"
+#include "libc/runtime/runtime.h"
+
+/**
+ * Returns current working directory.
+ *
+ * If the `PWD` environment variable is set, and it's correct, then
+ * that'll be returned in its exact canonical or non-canonical form
+ * instead of calling getcwd().
+ *
+ * @return pointer that must be free()'d, or NULL w/ errno
+ */
+char *get_current_dir_name(void) {
+  const char *res;
+  struct stat st1, st2;
+  if ((res = getenv("PWD")) && *res &&  //
+      !stat(res, &st1) &&               //
+      !stat(".", &st2) &&               //
+      st1.st_dev == st2.st_dev &&       //
+      st1.st_ino == st2.st_ino) {
+    return strdup(res);
+  } else {
+    return getcwd(0, 0);
+  }
+}