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/runtime/vfork.S

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/dce.h"
 #include "libc/intrin/strace.internal.h"
+#include "libc/thread/tls.h"
 #include "libc/macros.internal.h"
 .privileged
 
@@ -26,7 +27,10 @@
 //	This is the same as fork() except it's optimized for the case
 //	where the caller invokes execve() immediately afterwards. You
 //	can also call functions like close(), dup2(), etc. Call _exit
-//	but don't call exit. Look for vforksafe function annotations.
+//	but don't call exit. Look for vforksafe function annotations,
+//	For example pthread mutexes are @vforksafe because they don't
+//	do anything in a vfork()'d child process. TLS memory must not
+//	be disabled (it's enabled by default) since vfork() needs it.
 //
 //	Do not make the assumption that the parent is suspended until
 //	the child terminates since this impl calls fork() on Windows,
@@ -34,8 +38,10 @@
 //
 //	@return	pid of child process or 0 if forked process
 //	@returnstwice
+//	@threadsafe
 //	@vforksafe
-vfork:	xor	%edi,%edi		# dwCreationFlags
+vfork:	call	__require_tls
+	xor	%edi,%edi		# dwCreationFlags
 #ifdef __SANITIZE_ADDRESS__
 	jmp	fork			# TODO: asan and vfork don't mix?
 	.endfn	vfork,globl
@@ -56,45 +62,31 @@ vfork:	xor	%edi,%edi		# dwCreationFlags
 	ezlea	.Llog,di
 	call	__stracef
 #endif /* SYSDEBUG */
-	mov	__NR_vfork(%rip),%eax
-	mov	__errno(%rip),%r8d	# avoid question of @vforksafe errno
+	mov	%fs:0,%r9		# get thread information block
+	mov	0x3c(%r9),%r8d		# avoid question of @vforksafe errno
 	pop	%rsi			# saves return address in a register
+	mov	__NR_vfork(%rip),%eax
 #if SupportsBsd()
-	testb	IsBsd()
-	jnz	vfork.bsd
+	clc
 #endif
 	syscall
+#if SupportsBsd()
+	jnc	0f
+	neg	%rax
+0:
+#endif
 	push	%rsi			# note it happens twice in same page
-#if SupportsLinux()
 	cmp	$-4095,%eax
 	jae	systemfive_error
-#endif
-0:	mov	%r8d,__errno(%rip)
-	ezlea	__vforked,di
+	mov	%r8d,0x3c(%r9)		# restore errno
 	test	%eax,%eax
-	jz	1f
-	decl	(%rdi)
-	jns	2f			# openbsd doesn't actually share mem
-1:	incl	(%rdi)
-2:	ret
+	jnz	.Lprnt
+.Lchld:	orb	$TIB_FLAG_VFORKED,0x40(%r9)
+	ret
+.Lprnt:	andb	$~TIB_FLAG_VFORKED,0x40(%r9)
+	ret
 	.endfn	vfork,globl
 
-#if SupportsBsd()
-vfork.bsd:
-	syscall
-	push	%rsi
-	jc	systemfive_errno
-#if SupportsXnu()
-	testb	IsXnu()
-	jz	0b
-	neg	%edx			# edx is 0 for parent and 1 for child
-	not	%edx			# eax always returned with childs pid
-	and	%edx,%eax
-#endif /* XNU */
-	jmp	0b
-	.endfn	vfork.bsd
-#endif /* BSD */
-
 #ifdef SYSDEBUG
 	.rodata.str1.1
 .Llog:	.ascii	STRACE_PROLOGUE