Minor POSIX overhaul

Hi all,

I modified the POSIX module so that Scheme code instead of C code
manages PID/errno/signal objects. So, where before posix_kill took
a PID record and a signal record, now it takes ints. I think the
comments hinted towards something along these lines. This removes
a good chunk of awkward (in my view) Scheme-written-in-C code.

I put up a repo at bitbucket.org/wnoble/scheme48. I've only tested
the code lightly, and thought lightly about the general approach,
so I definitely would appreciate any suggestions!

Best,
Will