conf.c: New function Read_Config_File()

This function reads in and parses a single configuration file.
Adjust all other functions to display the correct name of the
file being read in currently.
Implement new configuration option "IncludeDir"

The option "IncludeDir" in the [Options] section can be used
to specify a directory which can contain further configuration
files and configuration file snippets matching the pattern
"*.conf" that should be read in after the main configuration
file ("ngircd.conf" by default) has been parsed.

Closes bug #157.
"IncludeDir" can't be overwritten by included files
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!