Show a warning on startup if config file is not a full path

Show a warning on startup if config file is not a full path

ngIRCd is a long-running process and changes its working directory to "/" to
not block mounted filesystems and the like when running as daemon ("not in the
foreground"); therefore the path to the configuration file must be relative to
"/" (or the chroot() directory), which basically is "not relative", to ensure
that "kill -HUP" and the "REHASH" command work as expected later on.

This fixes parts of bug #127.