bark if file descriptor goes above FD_SETSIZE. from openbsd.
XXX needs more checking.
XXX what is tab stop size for this code? need more consistency...
XXX we should really remove #ifdef CRAY, UNICOS5 and such.
we just cannot read it through.
to keep stuff out of the environment on the way to exec (we already have
one in the state machine that keeps them from arriving over the wire, so
this should be redundant, but it'll make any further updates easier to
have it present).
also, RCS Id police.
