IncludeDir: default to "$SYSCONFDIR/ngircd.conf.d"

Now "IncludeDir" defaults to "$SYSCONFDIR/ngircd.conf.d" instead
of no directory, but a missing directory is only reported as an
error if it has explicitely configured in the main configuration
file and simply ignored otherwise.

Therefore it is now possible not to touch the default (sample)
configuration file at all, and set all distribution and/or system
specific configuration options in "*.conf" files stored in
"$SYSCONFDIF/ngircd.conf.d/".

Thanks to "Elmasloco" for the idea!