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/runtime/getinterpreterexecutablename.c

@@ -51,6 +51,9 @@ char *GetInterpreterExecutableName(char *p, size_t n) {
   if (n < 2) {
     errno = ENAMETOOLONG;
   } else if (IsWindows() || IsXnu()) {
+    // TODO(jart): Does XNU guarantee argv[0] is legit?
+    //             Otherwise we should return NULL.
+    //             What about OpenBSD?
     if (strlen(GetProgramExecutableName()) < n) {
       strcpy(p, GetProgramExecutableName());
       return p;

libc/runtime/getsymboltable.c

@@ -96,7 +96,13 @@ static struct SymbolTable *GetSymbolTableFromZip(struct Zipos *zipos) {
  * @note This code can't depend on dlmalloc()
  */
 static struct SymbolTable *GetSymbolTableFromElf(void) {
-  return OpenSymbolTable(FindDebugBinary());
+  int e;
+  const char *s;
+  if ((s = FindDebugBinary())) {
+    return OpenSymbolTable(s);
+  } else {
+    return 0;
+  }
 }
 
 /**
@@ -117,7 +123,7 @@ static struct SymbolTable *GetSymbolTableFromElf(void) {
  * Function tracing is disabled throughout the duration of this call.
  * Backtraces and other core runtime functionality depend on this.
  *
- * @return symbol table, or NULL w/ errno on first call
+ * @return symbol table, or NULL if not found
  */
 struct SymbolTable *GetSymbolTable(void) {
   struct Zipos *z;

libc/runtime/mmap.c

@@ -323,7 +323,7 @@ static noasan inline void *Mmap(void *addr, size_t size, int prot, int flags,
 
   clashes = OverlapsImageSpace(p, size) || OverlapsExistingMapping(p, size);
 
-  if ((flags & MAP_FIXED_NOREPLACE) && clashes) {
+  if ((flags & MAP_FIXED_NOREPLACE) == MAP_FIXED_NOREPLACE && clashes) {
     STRACE("noreplace overlaps existing");
     return VIP(eexist());
   }
@@ -464,14 +464,21 @@ static noasan inline void *Mmap(void *addr, size_t size, int prot, int flags,
  *       compile-time checks to ensure some char[8192] vars will not
  *       create an undetectable overflow into another thread's stack
  *     Your `flags` may optionally bitwise or any of the following:
- *     - `MAP_FIXED` in which case `addr` becomes more than a hint
- *     - `MAP_FIXED_NOREPLACE` to protect existing maps (Linux-only)
  *     - `MAP_ANONYMOUS` in which case `fd == -1` should be the case
+ *     - `MAP_FIXED` in which case `addr` becomes more than a hint
+ *     - `MAP_FIXED_NOREPLACE` to protect existing mappings; this is
+ *       always polyfilled by mmap() which tracks its own memory and
+ *       removed before passing to the kernel, in order to support
+ *       old versions; if you believe mappings exist which only the
+ *       kernel knows, then this flag may be passed to sys_mmap() on
+ *       Linux 4.17+ and FreeBSD (where it has multiple bits)
  *     - `MAP_CONCEAL` is FreeBSD/NetBSD/OpenBSD-only
  *     - `MAP_NORESERVE` is Linux/XNU/NetBSD-only
- *     - `MAP_LOCKED` is Linux-only
- *     - `MAP_POPULATE` is Linux-only
+ *     - `MAP_POPULATE` is Linux/FreeBSD-only
  *     - `MAP_NONBLOCK` is Linux-only
+ *     - `MAP_NOSYNC` is FreeBSD-only
+ *     - `MAP_INHERIT` is NetBSD-only
+ *     - `MAP_LOCKED` is Linux-only
  * @param fd is an open()'d file descriptor, whose contents shall be
  *     made available w/ automatic reading at the chosen address and
  *     must be -1 if MAP_ANONYMOUS is specified