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/sem_init.c

@@ -16,6 +16,8 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/dce.h"
 #include "libc/intrin/atomic.h"
 #include "libc/limits.h"
 #include "libc/sysv/errfuns.h"
@@ -25,15 +27,25 @@
 /**
  * Initializes unnamed semaphore.
  *
+ * Calling sem_init() on an already initialized semaphore is undefined.
+ *
  * @param sem should make its way to sem_destroy() if this succeeds
- * @param pshared if semaphore may be shared between processes
+ * @param pshared if semaphore may be shared between processes, provided
+ *     `sem` is backed by `mmap(MAP_ANONYMOUS | MAP_SHARED)` memory
  * @param value is initial count of semaphore
  * @return 0 on success, or -1 w/ errno
  * @raise EINVAL if `value` exceeds `SEM_VALUE_MAX`
+ * @raise EPERM on OpenBSD if `pshared` is true
  */
 int sem_init(sem_t *sem, int pshared, unsigned value) {
   if (value > SEM_VALUE_MAX) return einval();
+  // OpenBSD MAP_ANONYMOUS|MAP_SHARED memory is kind of busted.
+  // The OpenBSD implementation of sem_init() also EPERMs here.
+  if (IsOpenbsd() && pshared) return eperm();
+  sem->sem_magic = SEM_MAGIC_UNNAMED;
   atomic_store_explicit(&sem->sem_value, value, memory_order_relaxed);
-  sem->sem_pshared = pshared;
+  sem->sem_pshared = !!pshared;
+  sem->sem_pid = getpid();
+  sem->sem_waiters = 0;
   return 0;
 }