#12 support nommu linux systems (no fork)

[Mike Frysinger]

nommu linux systems lack fork(), so the exp_command.c needs a little work

i updated the configure script to check for the fork function, and then exp_command.c does three things:
- call vfork() in Exp_SpawnObjCmd when fork() is missing
- call _exit() with vfork() and exit() with fork()
- disable the "fork" command completely since there's no real (or easy) way to support it

[Mike Frysinger]

no-fork patch