Doing the vfork work on ash on a branch to try to shake out the
problems before I expose everyone to them.  This checkin represents
a merge of the prior work, which I backed out a while ago, to the
HEAD only and does not incorporate any additional bugfixes.  The
additional bugfixes and code-cleanup will occur in later checkins.

For reference the patches that were used are:
cvs diff -kk -r1.51 -r1.55 eval.c  | patch
cvs diff -kk -r1.27 -r1.28 exec.c  | patch
cvs diff -kk -r1.15 -r1.16 exec.h  | patch
cvs diff -kk -r1.32 -r1.33 input.c | patch
cvs diff -kk -r1.10 -r1.11 input.h | patch
cvs diff -kk -r1.32 -r1.35 jobs.c  | patch
cvs diff -kk -r1.9  -r1.11 jobs.h  | patch
cvs diff -kk -r1.36 -r1.37 main.c  | patch
cvs diff -kk -r1.20 -r1.21 redir.c | patch
cvs diff -kk -r1.10 -r1.11 redir.h | patch
cvs diff -kk -r1.10 -r1.12 shell.h | patch
cvs diff -kk -r1.22 -r1.23 trap.c  | patch
cvs diff -kk -r1.12 -r1.13 trap.h  | patch
cvs diff -kk -r1.23 -r1.24 var.c   | patch
cvs diff -kk -r1.16 -r1.17 var.h   | patch

All other changes were simply the resolution of the resulting
conflicts, which occured only in the merge of jobs.c.

Begins to address PR: bin/5475