Avoid misidentifying unbracketed IPv6 literals as host:port.

Avoid misidentifying unbracketed IPv6 literals as host:port.

Both GUI PuTTY front ends have a piece of logic whereby a string is
interpreted as host:port if there's _one_ colon in it, but if there's
more than one colon then it's assumed to be an IPv6 literal with no
trailing port number. This permits the PuTTY command line to take
strings such as 'host', 'host:22' or '[::1]:22', but also cope with a
bare v6 literal such as '::1'.

This logic is also required in the two Plink front ends and in the
processing of CONF_loghost for host key indexing in ssh.c, but was
missing in all those places. Add it.

[originally from svn r10121]