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/nt/kernel32/GetNamedPipeInfo.S

@@ -0,0 +1,18 @@
+#include "libc/nt/codegen.h"
+.imp	kernel32,__imp_GetNamedPipeInfo,GetNamedPipeInfo
+
+	.text.windows
+        .ftrace1
+GetNamedPipeInfo:
+        .ftrace2
+#ifdef __x86_64__
+	push	%rbp
+	mov	%rsp,%rbp
+	mov	__imp_GetNamedPipeInfo(%rip),%rax
+	jmp	__sysv2nt6
+#elif defined(__aarch64__)
+	mov	x0,#0
+	ret
+#endif
+	.endfn	GetNamedPipeInfo,globl
+	.previous