Add certificate fingerprint support
Simplify MOTD handling

Merge Show_MOTD_Start, Show_MOTD_Sendline and Show_MOTD_End into
IRC_Show_MOTD.
Fix spelling
Improved client announcement

Move Announce_User() to client.c and rename it to Client_Announce().
Use this in cb_introduceClient() instead of duplicating the code.
This fix the certificate fingerprint announcement for new clients.
Also ensure the certificate fingerprint is only announced if the
client supports it (`M' flag).