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/runtime/enable_tls.c

@@ -102,11 +102,11 @@ textstartup void __enable_tls(void) {
 
   // Here's the layout we're currently using:
   //
-  //         .balign PAGESIZE
+  //         .balign 4096
   //     _tdata_start:
   //         .tdata
   //         _tdata_size = . - _tdata_start
-  //         .balign PAGESIZE
+  //         .balign 4096
   //     _tbss_start:
   //     _tdata_start + _tbss_offset:
   //         .tbss