with-authinfo-kludge: Do job-control to make interrupt characters work.

with-authinfo-kludge: Do job-control to make interrupt characters work.

Terminal-based newsreaders use terminal interrupt characters for their
own purposes, i.e. slrn(1) arranges that C-g is the `VINTR' character
(i.e., it sends `SIGINT' to the foreground process group).  This is bad
unless we take evasive action: it makes `slrn' go ding and cancel some
UI action, but it also wipes out `with-authinfo-kludge' and its various
helper processes, and brings the parent shell back into the foreground
where it fights `slrn' over the terminal.

Avoid this by running the client in a separate process group, pushing it
into the foreground, and proxying signals and foregroundness back and
forth to keep the shell happy.  It's all a bit ugly and desperate, but
it works well in practice and doesn't involve ignoring signals
everywhere (which was my first attempt at solving this bug).