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/describesocketprotocol.c

@@ -27,6 +27,7 @@ const char *(DescribeSocketProtocol)(char buf[12], int family) {
   if (family == IPPROTO_UDP) return "IPPROTO_UDP";
   if (family == IPPROTO_RAW) return "IPPROTO_RAW";
   if (family == IPPROTO_IPV6) return "IPPROTO_IPv6";
+  if (family == IPPROTO_ICMPV6) return "IPPROTO_ICMPV6";
   FormatInt32(buf, family);
   return buf;
 }