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/g_fds_init.S

@@ -21,6 +21,9 @@
 	.init.start 305,_init_g_fds
 	push	%rdi
 	push	%rsi
+	mov	%r12d,%edi			// argc
+	mov	%r13,%rsi			// argv
+	mov	%r14,%rdx			// environ
 	call	__init_fds
 	pop	%rsi
 	pop	%rdi