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/mprotect.c

@@ -23,6 +23,8 @@
 #include "libc/intrin/likely.h"
 #include "libc/intrin/strace.internal.h"
 #include "libc/runtime/internal.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sysv/consts/auxv.h"
 #include "libc/sysv/consts/prot.h"
 #include "libc/sysv/errfuns.h"
 
@@ -41,7 +43,7 @@ int mprotect(void *addr, size_t size, int prot) {
     rc = einval();  // unix checks prot before checking size
   } else if (!size) {
     return 0;  // make new technology consistent with unix
-  } else if (UNLIKELY((intptr_t)addr & 4095)) {
+  } else if (UNLIKELY((intptr_t)addr & (getauxval(AT_PAGESZ) - 1))) {
     rc = einval();  // unix checks prot before checking size
   } else if (!IsWindows()) {
     rc = sys_mprotect(addr, size, prot);