Committed initial (system* "foo" "bar") code.

[Rob Browning]

After reading and thinking about all the comments, I decided on a C
implementation so that people can easily call scm_system_star from C
if they need to, and so that we can have more careful control over the
system state if needed.

The current version is careful to allocate the args before the fork,
and follows the libc and perl convention of blocking SIGINT and
SIGQUIT so that killing the child won't kill guile (in perl's similar
function).  That's all the current version deals with.  I decided to
start with the current code, which covers the stuff that obviously has
to be done, and then make fixes as we figure out what else we need.
(Docs have been added to posix.texi as well.)

Thanks.
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
GPG starting 2002-11-03 = 14DD 432F AE39 534D B592  F9A0 25C8 D377 8C7E 73A4