network-functions.pl: Introduce convert_to_cidr_or_mask() function.

network-functions.pl: Introduce convert_to_cidr_or_mask() function.

This function is used to convert a single host address or network into
different formats.

If a single host address is given, it will be converted
to a plain address without any notation details.

When a single host address is given and a format is specified ("cidr"
or "mask") the host address with the notation details will be returned.

In case a network is given the output will be formated into the desired
format ("cidr" or "mask"). Unless a special output format is given for
a network it defaults to CIDR.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>