script/lei: background ourselves on MUA/pager exec

script/lei: background ourselves on MUA/pager exec

This ought to give the MUA or pager exclusive access to the
controlling terminal.  The downside is we can only exec the
pager or MUA once per invocation, but I can't imagine a valid
case for running those things multiple times, either.

Note: I'm no expert when it comes to terminal control matters,
but this allows Ctrl-Z-ed mutt instance to come back and is
a nice code reduction, as well.