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/thread/posixthread.internal.h

@@ -1,6 +1,7 @@
 #ifndef COSMOPOLITAN_LIBC_THREAD_POSIXTHREAD_INTERNAL_H_
 #define COSMOPOLITAN_LIBC_THREAD_POSIXTHREAD_INTERNAL_H_
 #include "libc/calls/struct/sched_param.h"
+#include "libc/calls/struct/sigset.h"
 #include "libc/runtime/runtime.h"
 #include "libc/thread/thread.h"
 #include "libc/thread/tls.h"
@@ -15,6 +16,8 @@ COSMOPOLITAN_C_START_
  * @fileoverview Cosmopolitan POSIX Thread Internals
  */
 
+typedef void (*atfork_f)(void);
+
 // LEGAL TRANSITIONS             ┌──> TERMINATED
 // pthread_create ─┬─> JOINABLE ─┴┬─> DETACHED ──> ZOMBIE
 //                 └──────────────┘
@@ -73,6 +76,7 @@ struct PosixThread {
   struct CosmoTib *tib;    // middle of tls allocation
   jmp_buf exiter;          // for pthread_exit
   pthread_attr_t attr;
+  sigset_t sigmask;
   struct _pthread_cleanup_buffer *cleanup;
 };
 
@@ -82,8 +86,7 @@ hidden extern uint64_t _pthread_key_usage[(PTHREAD_KEYS_MAX + 63) / 64];
 hidden extern pthread_key_dtor _pthread_key_dtor[PTHREAD_KEYS_MAX];
 hidden extern _Thread_local void *_pthread_keys[PTHREAD_KEYS_MAX];
 
-void _pthread_atfork(int) hidden;
-void _pthread_funlock(pthread_mutex_t *, int) hidden;
+int _pthread_atfork(atfork_f, atfork_f, atfork_f) hidden;
 int _pthread_reschedule(struct PosixThread *) hidden;
 int _pthread_setschedparam_freebsd(int, int, const struct sched_param *) hidden;
 void _pthread_free(struct PosixThread *) hidden;
@@ -91,9 +94,15 @@ void _pthread_cleanup(struct PosixThread *) hidden;
 void _pthread_ungarbage(void) hidden;
 void _pthread_wait(struct PosixThread *) hidden;
 void _pthread_zombies_add(struct PosixThread *) hidden;
+void _pthread_zombies_purge(void) hidden;
 void _pthread_zombies_decimate(void) hidden;
 void _pthread_zombies_harvest(void) hidden;
 void _pthread_key_destruct(void *[PTHREAD_KEYS_MAX]) hidden;
+void _pthread_key_lock(void) hidden;
+void _pthread_key_unlock(void) hidden;
+void _pthread_onfork_prepare(void) hidden;
+void _pthread_onfork_parent(void) hidden;
+void _pthread_onfork_child(void) hidden;
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */