Write tests for new APE loader and fix bugs

- Add FreeBSD-specific mmap() flags
- Reduce size of the APE loader from 8kb to 4kb
- Work towards fixing the Makefile build on WSL
- Automate testing of APE no-modify-self behaviors
- Make the ape.S shell script code cleaner and tinier
- Improve the APE sanity check to test behavior better
- Fixed issue with ShowCrashReports() sigaltstack() on BSDs
- Delete symbols for S_MODE magnums which wasted compile time

If you checked out yesterday's APE commit, please run:

    rm -f /usr/bin/ape o/tmp/ape /tmp/ape "${TMPDIR:-/tmp}/ape"

Because this change fixes certain aspects of the new ABI. We don't have
automated migrations for APE loader versions yet. Thanks! You can also
download prebuilt binaries here:

- https://justine.lol/ape.elf    (Linux/FreeBSD/NetBSD/OpenBSD)
- https://justine.lol/ape.macho  (Apple)

Install the appropriate one as `/usr/bin/ape`.

libc/calls/getprogramexecutablename.greg.c

@@ -34,12 +34,22 @@
 
 char program_executable_name[PATH_MAX];
 
+static inline char *StrCat(char buf[PATH_MAX], const char *a, const char *b) {
+  char *p, *e;
+  p = buf;
+  e = buf + PATH_MAX;
+  while (*a && p < e) *p++ = *a++;
+  while (*b && p < e) *p++ = *b++;
+  return buf;
+}
+
 static inline void GetProgramExecutableNameImpl(char *p, char *e) {
   char *q;
   ssize_t rc;
   size_t i, n;
   union {
     int cmd[4];
+    char path[PATH_MAX];
     char16_t path16[PATH_MAX];
   } u;
 
@@ -61,7 +71,11 @@ static inline void GetProgramExecutableNameImpl(char *p, char *e) {
     return;
   }
 
-  if (__argc && (q = __argv[0]) && !sys_faccessat(AT_FDCWD, q, F_OK, 0)) {
+  // if argv[0] exists then turn it into an absolute path. we also try
+  // adding a .com suffix since the ape auto-appends it when resolving
+  if (__argc && (((q = __argv[0]) && !sys_faccessat(AT_FDCWD, q, F_OK, 0)) ||
+                 ((q = StrCat(u.path, __argv[0], ".com")) &&
+                  !sys_faccessat(AT_FDCWD, q, F_OK, 0)))) {
     if (*q != '/') {
       if (q[0] == '.' && q[1] == '/') {
         q += 2;
@@ -78,12 +92,12 @@ static inline void GetProgramExecutableNameImpl(char *p, char *e) {
     return;
   }
 
+  // if argv[0] doesn't exist, then fallback to interpreter name
   if ((rc = sys_readlinkat(AT_FDCWD, "/proc/self/exe", p, e - p - 1)) > 0 ||
       (rc = sys_readlinkat(AT_FDCWD, "/proc/curproc/file", p, e - p - 1)) > 0) {
     p[rc] = 0;
     return;
   }
-
   if (IsFreebsd() || IsNetbsd()) {
     u.cmd[0] = CTL_KERN;
     u.cmd[1] = KERN_PROC;
@@ -101,7 +115,7 @@ static inline void GetProgramExecutableNameImpl(char *p, char *e) {
 }
 
 /**
- * Returns absolute path of executable.
+ * Returns absolute path of program.
  */
 char *GetProgramExecutableName(void) {
   int e;