Implement an additional flag for hostile networks safe to drop

Implement an additional flag for hostile networks safe to drop

This patch implements an additional flag intended for networks and
Autonomous Systems being considered hostile. While libloc does not and
should not be an opinionated database, reality shows it is being used
this way.

Hereby, we assign "XD" (drop) as a custom country code for networks
being flagged this way. According to ISO, "XA" to "XZ" are reserved for
"user-assgined codes" (http://www.iso.org/glossary-for-iso-3166.html),
so this is a safe thing to do.

This patch does not interfere with "A1" to "A3", which we currently
assign outside standardised country code ranges for historical reasons.

Neither does it specify any policy or source for tagging networks with a
"drop" flag. Doing so is beyond the scope of this - technical -
approach.

To avoid confusions with the SQL "DROP" command, "is_drop" will be used
as a column name for database operations.

Thanks to Michael for his remarks and ideas during the run-up.

Cc: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>