make available a namespace-safe vfork, if supported

this may be useful to posix_spawn..?

src/process/vfork.c

@@ -1,8 +1,11 @@
 #include <unistd.h>
 #include "syscall.h"
+#include "libc.h"
 
-pid_t vfork(void)
+pid_t __vfork(void)
 {
 	/* vfork syscall cannot be made from C code */
 	return syscall(SYS_fork);
 }
+
+weak_alias(__vfork, vfork);