Tweak to r7635: when changing between protocols in the configuration dialog

Tweak to r7635: when changing between protocols in the configuration dialog
and avoiding trashing a non-default port, don't treat a port of 0 as special;
this was causing defaults to "freeze in" for sequences of clicks like SSH,
Raw, Telnet.
Arrange that a port of 0 (which in a backend indicates no default) is displayed
as a blank in the port box, to make this less jarring.

git-svn-id: http://svn.tartarus.org/sgt/putty@9077 cda61777-01e9-0310-a592-d414129be87e