libroot: Implement the general case of posix_spawn using load_image.

The goal here is to avoid potentially expensive fork()ing.

The time for a fork() is (for a process with no real heap usage
and thus few areas) 300-400us on my system. load_image() takes
3000us (3ms) or so, but this of course includes exec() time.

Overall, for compiling HaikuDepot (with a tweaked jam to use
posix_spawn on Haiku, not just on Linux) there is a slight
decrease in time:

before:
real 1m21.727s
user 1m2.131s
sys  0m43.029s

after:
real 1m19.472s
user 1m1.752s
sys  0m41.740s

Which is probably within the realm of "noise", so more benchmarks
are needed. Likely if we tweak our jam usage to not need as many
shells when running commands, this would be a much more noticeable
change.

Change-Id: I217f2476b1ed9aa18322b3c2bc8986571d89549a

headers/private/libroot/libroot_private.h

@@ -33,6 +33,8 @@ status_t __test_executable(const char *path, char *invoker);
 status_t __flatten_process_args(const char* const* args, int32 argCount,
 			const char* const* env, int32* envCount, const char* executablePath,
 			char*** _flatArgs, size_t* _flatSize);
+thread_id __load_image_at_path(const char* path, int32 argCount,
+			const char **args, const char **environ);
 void _call_atexit_hooks_for_range(addr_t start, addr_t size);
 void __init_env(const struct user_space_program_args *args);
 void __init_env_post_heap(void);