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/sysv/consts/c.h

@@ -1,25 +1,29 @@
 #ifndef COSMOPOLITAN_LIBC_SYSV_CONSTS_CPIO_H_
 #define COSMOPOLITAN_LIBC_SYSV_CONSTS_CPIO_H_
 
-#define C_IXOTH  0000001
-#define C_IWOTH  0000002
-#define C_IROTH  0000004
-#define C_IXGRP  0000010
-#define C_IWGRP  0000020
-#define C_IRGRP  0000040
-#define C_IXUSR  0000100
-#define C_IWUSR  0000200
-#define C_IRUSR  0000400
-#define C_ISVTX  0001000
-#define C_ISGID  0002000
-#define C_ISUID  0004000
-#define C_ISFIFO 0010000
-#define C_ISCHR  0020000
-#define C_ISDIR  0040000
-#define C_ISBLK  0060000
-#define C_ISREG  0100000
-#define C_ISCTG  0110000
-#define C_ISLNK  0120000
+#define MAGIC "070707"
+
+#define C_IRUSR 000400
+#define C_IWUSR 000200
+#define C_IXUSR 000100
+#define C_IRGRP 000040
+#define C_IWGRP 000020
+#define C_IXGRP 000010
+#define C_IROTH 000004
+#define C_IWOTH 000002
+#define C_IXOTH 000001
+
+#define C_ISUID 004000
+#define C_ISGID 002000
+#define C_ISVTX 001000
+
+#define C_ISBLK  060000
+#define C_ISCHR  020000
+#define C_ISDIR  040000
+#define C_ISFIFO 010000
 #define C_ISSOCK 0140000
+#define C_ISLNK  0120000
+#define C_ISCTG  0110000
+#define C_ISREG  0100000
 
 #endif /* COSMOPOLITAN_LIBC_SYSV_CONSTS_CPIO_H_ */