Use shared net lock for DIOCGETIFACES

Use shared net lock for DIOCGETIFACES

snmpd(8) and 'pfctl -s Interfaces' dump pf's internal list of interfaces.

pf's internal interface list is completely protected by the pf lock,
pf lock assertions since pf_if.c r1.110 from over a week ago support this.

pfi_*() iterate over net lock protected if_groups lists, but only to read,
so downgrade from exclusive write net lock to a shared read-only one.

Feedback mvs
OK sashan