implement _Fork and refactor fork using it

the _Fork interface is defined for future issue of POSIX as the
outcome of Austin Group issue 62, which drops the AS-safety
requirement for fork, and provides an AS-safe replacement that does
not run the registered atfork handlers.

src/process/_Fork.c

@@ -1,23 +1,17 @@
 #include <unistd.h>
-#include <string.h>
 #include <signal.h>
 #include "syscall.h"
 #include "libc.h"
 #include "lock.h"
 #include "pthread_impl.h"
 
-static void dummy(int x)
-{
-}
-
-weak_alias(dummy, __fork_handler);
+static void dummy(int x) { }
 weak_alias(dummy, __aio_atfork);
 
-pid_t fork(void)
+pid_t _Fork(void)
 {
 	pid_t ret;
 	sigset_t set;
-	__fork_handler(-1);
 	__block_all_sigs(&set);
 	__aio_atfork(-1);
 	LOCK(__abort_lock);
@@ -39,6 +33,5 @@ pid_t fork(void)
 	UNLOCK(__abort_lock);
 	__aio_atfork(!ret);
 	__restore_sigs(&set);
-	__fork_handler(!ret);
 	return __syscall_ret(ret);
 }

src/process/fork.c

@@ -0,0 +1,13 @@
+#include <unistd.h>
+#include "libc.h"
+
+static void dummy(int x) { }
+weak_alias(dummy, __fork_handler);
+
+pid_t fork(void)
+{
+	__fork_handler(-1);
+	pid_t ret = _Fork();
+	__fork_handler(!ret);
+	return ret;
+}