LibC: Add posix_spawn()!

All the file actions stuff is still missing for now,
as is POSIX_SPAWN_SETSCHEDULER (not sure what that's
supposed to do) and POSIX_SPAWN_RESETIDS.

Implemented in userspace for now. Once there are users,
it'll likely make sense to make this a syscall for
performance reasons.

A simple test program of the form

    extern char **environ;
    int main(int argc, char* argv[])
    {
        pid_t pid;
        char* args[] = { "ls", NULL };
        posix_spawnp(&pid, "ls", nullptr, nullptr, args, environ);
    }

works fine.

Libraries/LibC/CMakeLists.txt

@@ -25,6 +25,7 @@ set(LIBC_SOURCES
     serenity.cpp
     setjmp.S
     signal.cpp
+    spawn.cpp
     stat.cpp
     stdio.cpp
     stdlib.cpp