The centralisation I did in commit e3796cb77 introduced a foolish
sense error, in which I was supposed to be treating an unbracketed
colon in CONF_hostname as separating host name from a port-number-
shaped suffix if it _was_ the only one, and instead, accidentally did
so if it _wasn't_.
A more or less identical piece of code to sanitise the CONF_host
string prior to session launch existed in Windows PuTTY and both
Windows and Unix Plink. It's long past time it was centralised.
While I'm here, I've added a couple of extra comments in the
centralised version, including one that - unfortunately - tries _but
fails_ to explain why a string of the form "host.name:1234" doesn't
get the suffix moved into CONF_port the way "user@host" moves the
prefix into CONF_username. Commit c1c1bc471 is the one I'm referring
to in the comment, and unfortunately it has an unexplained one-liner
log message from before I got into the habit of being usefully
verbose.
