Implement pthread_atfork()

If threads are being used, then fork() will now acquire and release and
runtime locks so that fork() may be safely used from threads. This also
makes vfork() thread safe, because pthread mutexes will do nothing when
the process is a child of vfork(). More torture tests have been written
to confirm this all works like a charm. Additionally:

- Invent hexpcpy() api
- Rename nsync_malloc_() to kmalloc()
- Complete posix named semaphore implementation
- Make pthread_create() asynchronous signal safe
- Add rm, rmdir, and touch to command interpreter builtins
- Invent sigisprecious() and modify sigset functions to use it
- Add unit tests for posix_spawn() attributes and fix its bugs

One unresolved problem is the reclaiming of *NSYNC waiter memory in the
forked child processes, within apps which have threads waiting on locks

libc/assert.h

@@ -17,7 +17,7 @@ void __assert_fail(const char *, const char *, int) hidden relegated;
 #endif
 
 #ifdef __GNUC__
-#ifndef TINY
+#if !defined(TINY) && !defined(MODE_DBG)
 /**
  * Specifies expression can't possibly be false.
  *
@@ -36,7 +36,7 @@ void __assert_fail(const char *, const char *, int) hidden relegated;
  * In `MODE=tiny` these assertions are redefined as undefined behavior.
  */
 #define _npassert(x)                 \
-  ({                                 \
+  __extension__({                    \
     if (__builtin_expect(!(x), 0)) { \
       notpossible;                   \
     }                                \
@@ -63,7 +63,7 @@ void __assert_fail(const char *, const char *, int) hidden relegated;
  * that's capable of debugging this macro.
  */
 #define _unassert(x)                 \
-  ({                                 \
+  __extension__({                    \
     if (__builtin_expect(!(x), 0)) { \
       unreachable;                   \
     }                                \