Commands.txt: Update more descriptions

Update description of "CONNECT", "DIE", "DISCONNECT", "GLINE", "KILL",
"KLINE", "OPER", "REHASH", "RESTART" and "WALLOPS".
Update irc-oper.c to use irc-macros.h
Commands.txt: Update more descriptions

Update description of "CHANINFO", "METADATA", "NJOIN", "SERVER", "SQUIT",
"GET", and "POST".
Update irc-server.c to use irc-macros.h

And add some comments :-)
Add certificate fingerprint support
Simplify MOTD handling

Merge Show_MOTD_Start, Show_MOTD_Sendline and Show_MOTD_End into
IRC_Show_MOTD.
Merge remote-tracking branch 'fgsch/certfp'

* fgsch/certfp:
  Simplify MOTD handling
  Add certificate fingerprint support