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/calls/fcntl-nt.c

@@ -321,8 +321,9 @@ static textwindows int sys_fcntl_nt_setfl(int fd, unsigned *flags, unsigned arg,
   //
   // - O_NONBLOCK     make read() raise EAGAIN
   // - O_NDELAY       same thing as O_NONBLOCK
+  // - O_ACCMODE      but has a minimal effect
   //
-  allowed = O_NONBLOCK;
+  allowed = O_ACCMODE | O_NONBLOCK;
   if (changed & ~allowed) {
     // the following access mode flags are supported, but it's currently
     // not possible to change them on windows.