Use set-key as command for setting a key

This commit changes the command for setting a key for a channel to
set-key. It also fixes the online help for this command.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Remove duplicated code for command handling

The handler for commands entered in the IRCrypt buffer and for the
/ircrypt contained almost identical code. There are only some additional
commands only allowed in the IRCrypt buffer like accept or decline.

This commit removes all of the duplicated code and just uses the same
methods for both types of commands.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>