Network: make whois reply more clear

Mention that it's possible that whoised user has channel list hiding
umode. This umode is usually +i, but I say "channel list hiding umode",
because +i is not always the umode which hides user list.

For example Quakenet always forces mode +i to every user and it cannot
be unset, but channel list is still visible. Some other IRCds can have
different modes to hide channel lists than +i.
Merge pull request #936 from Mkaysi/network

Network: make whois reply more clear