Don't always add a colon to custom commands

The IRC spec mandates that spaces be used as separators between the
arguments for a command. However, the last parameter may itself contain
spaces, and to allow this, one can prefix that argument with a colon.

Instead of checking if the colon is needed, Quassel just always added
a colon to the last argument, which usually works fine. However, it broke
some uses of custom server commands.

This fix now checks if the last argument contains a space or starts with
a colon (which then needs to be escaped), and leaves it alone otherwise.

Fixes #1173 - thanks to Gunnar Beutner for providing the patch.
Fix round-robin for networks that support it

Some IRC networks perform round-robin by supplying a list of IP addresses
in their DNS records. However, Qt a) always uses the first address of that
list, and b) caches DNS records for a minute. This results that users who
connect at a roughly similar time (like at core startup) will all pick the
same IP for a given round-robin network address.

We now force Qt (simply by calling QHostInfo::fromName()) to perform a fresh
lookup every time it connects to an IRC network, which solves this problem.
However, this will not help you if your OS uses a system DNS cache that does
not reshuffle the list of IPs for repeated lookups.