process-util: add posix_spawn helper

This provides CLONE_VM + CLONE_VFORK semantics, so it is useful to
avoid CoW traps and other issues around doing work between fork()
and exec().

src/basic/process-util.h

@@ -210,3 +210,5 @@ int get_process_threads(pid_t pid);
 
 int is_reaper_process(void);
 int make_reaper_process(bool b);
+
+int posix_spawn_wrapper(const char *path, char *const *argv, char *const *envp, pid_t *ret_pid);