Fix a reconfig bug wrt inline-signing

Fix a reconfig bug wrt inline-signing

When doing 'rndc reconfig', named may complain about a zone not being
reusable because it has a raw version of the zone, and the new
configuration has not set 'inline-signing'. However, 'inline-signing'
may be implicitly true if a 'dnssec-policy' is used for the zone, and
the zone is not dynamic.

Improve the check in 'named_zone_reusable'.  Create a new function for
checking 'inline-signing' configuration that matches existing code in
'bin/named/server.c'.