Remove (redundant and O(n)) error checking on +L, we catch the errors on join nowdays...

Remove (redundant and O(n)) error checking on +L, we catch the errors on join nowdays anyway, so this was a throwback to the "old days".
Also, add two new restrictions on +L: target channel must exist, and setter *must* be opped on target channel.
Avoids redirect spamming and other nefarious things.
Additionally, reworded numerics a bit to make it more generic, as we may forward for more than just +l in the future.

git-svn-id: http://svn.inspircd.org/repository/trunk/inspircd@10699 e03df62e-2008-0410-955e-edbf42e46eb7