Make more fixes and improvements

- Remove PAGESIZE constant
- Fix realloc() documentation
- Fix ttyname_r() error reporting
- Make forking more reliable on Windows
- Make execvp() a few microseconds faster
- Make system() a few microseconds faster
- Tighten up the socket-related magic numbers
- Loosen restrictions on mmap() offset alignment
- Improve GetProgramExecutableName() with getenv("_")
- Use mkstemp() as basis for mktemp(), tmpfile(), tmpfd()
- Fix flakes in pthread_cancel_test, unix_test, fork_test
- Fix recently introduced futex stack overflow regression
- Let sockets be passed as stdio to subprocesses on Windows
- Improve security of bind() on Windows w/ SO_EXCLUSIVEADDRUSE

libc/intrin/describeframe.c

@@ -37,8 +37,6 @@ static const char *GetFrameName(int x) {
     return "automap";
   } else if (IsFixedFrame(x)) {
     return "fixed";
-  } else if (IsArenaFrame(x)) {
-    return "arena";
   } else if (IsStaticStackFrame(x)) {
     return "stack";
   } else if (IsGfdsFrame(x)) {