Added spawning capabilities

This is a quick hack written to get applications that use posix_spawn{p,}() (e.g. clang) to work correctly in a fakechroot.

It is almost exactly a copy and paste of execvp.c and execve.c.  If you have another idea for how to implement this without so much copying of code let me know.  I just was looking for this to compile and work.

I've tested it and can confirm that it works on my machine.