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/sock/socket.c

@@ -22,26 +22,31 @@
 #include "libc/sock/internal.h"
 #include "libc/sock/sock.h"
 #include "libc/sysv/consts/af.h"
+#include "libc/sysv/errfuns.h"
 
 /**
  * Creates new system resource for network communication, e.g.
  *
  *     int fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  *
- * @param family can be AF_UNIX, AF_INET, etc.
+ * @param family can be AF_UNIX, AF_INET, AF_INET6, etc.
  * @param type can be SOCK_STREAM (for TCP), SOCK_DGRAM (e.g. UDP), or
  *     SOCK_RAW (IP) so long as IP_HDRINCL was passed to setsockopt();
  *     and additionally, may be or'd with SOCK_NONBLOCK, SOCK_CLOEXEC
  * @param protocol can be IPPROTO_TCP, IPPROTO_UDP, or IPPROTO_ICMP
  * @return socket file descriptor or -1 w/ errno
- * @error ENETDOWN, EPFNOSUPPORT, etc.
+ * @raise EAFNOSUPPORT if `family` isn't supported by system or platform
  * @see libc/sysv/consts.sh
  * @asyncsignalsafe
  */
 int socket(int family, int type, int protocol) {
   int rc;
-  if (family == AF_UNSPEC) family = AF_INET;
-  if (!IsWindows()) {
+  if (family == AF_UNSPEC) {
+    family = AF_INET;
+  }
+  if (family == -1) {
+    rc = eafnosupport();
+  } else if (!IsWindows()) {
     rc = sys_socket(family, type, protocol);
   } else {
     rc = sys_socket_nt(family, type, protocol);