ngircd: change MOTD file handling

ngircd: change MOTD file handling

previously, the given MotdFile file was read whenever a client
requested it.

Change handling to read the MotdFile contents into memory once
during config file parsing.

Two side effects:
- changes to the MOTD file do not have any effect until ngircds
  configuration is reloaded
- MOTD file does no longer have to reside in the chroot directory
  (the MOTD contents will then not be re-read on reload in that case)