Try to set file descriptor limit to its maximum and show info on startup

Try to set file descriptor limit to its maximum and show info on startup

The number of possible parallel connections is limited by the process
file descriptor limit (among other things). Therefore try to upgrade the
current "soft" limit to its "hard" maximum (but limit it to 100000), and
show an information or even warning, wenn still less than the configured
"MaxConnections" setting.

Please note that ngIRCd and its linked libraries (like PAM) need file
descriptors not only for incoming and outgoing IRC connections, but for
reading files and inter-process communication, too! Therefore the actual
connection limit is _less_ than the file descriptor limit!

This introduces the new MAX_FD_LIMIT (100000) #define.