Unix Plink, Unix Pageant in server mode, Uppity, and the post-
connection form of PSFTP's command-line reading code all had very
similar loops in them, which run a pollwrapper and mediate between
that, timers, and toplevel callbacks. It's long past time the common
code between all of those became a reusable shared routine.
So, this commit introduces uxcliloop.c, and turns all the previous
copies of basically the same loop into a call to cli_main_loop with
various callback functions to configure the parts that differ.
