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/openat.c

@@ -132,6 +132,7 @@
  * @raise ENOTSUP if `file` is on zip file system and process is vfork()'d
  * @raise ENOSPC if file system is full when `file` would be `O_CREAT`ed
  * @raise EINTR if we needed to block and a signal was delivered instead
+ * @raise EEXIST if `O_CREAT|O_EXCL` are used and `file` already existed
  * @raise ECANCELED if thread was cancelled in masked mode
  * @raise ENOENT if `file` doesn't exist when `O_CREAT` isn't in `flags`
  * @raise ENOENT if `file` points to a string that's empty