Reinstate a piece of code accidentally removed in r9214, where Windows

Reinstate a piece of code accidentally removed in r9214, where Windows
PuTTY does not trim a colon suffix off the hostname if it contains
_more than one_ colon. This allows IPv6 literals to be entered.

(Really we need to do a much bigger revamp of all uses of hostnames to
arrange that square-bracketed IPv6 literals work consistently, but
this at least removes a regression over 0.62.)

[originally from svn r9983]
[r9214 == a1f3b7a358adaa7c2a98359cd0373aa823eeb14b]