debianpkg: create frr-snmp and frr-rpki-rtrlib

debianpkg: create frr-snmp and frr-rpki-rtrlib

This splits off SNMP and RPKI support so that users can install these
packages (with the appropriate dependencies) independently of main FRR.
It also obsoletes out the weird multi-variant package distribution we've
been doing for RPKI support.

The snmpd dependency is also changed to Recommends: on frr-snmp since
the frr-snmp package is essentially useless without snmpd.

Signed-off-by: David Lamparter <equinox@diac24.net>