ospfd: add support for NSSA Type-7 address ranges

ospfd: add support for NSSA Type-7 address ranges

Implement NSSA address ranges as specified by RFC 3101:

   NSSA border routers may be configured with Type-7 address ranges.
   Each Type-7 address range is defined as an [address,mask] pair.  Many
   separate Type-7 networks may fall into a single Type-7 address range,
   just as a subnetted network is composed of many separate subnets.
   NSSA border routers may aggregate Type-7 routes by advertising a
   single Type-5 LSA for each Type-7 address range.  The Type-5 LSA
   resulting from a Type-7 address range match will be distributed to
   all Type-5 capable areas.

Syntax:
  area A.B.C.D nssa range A.B.C.D/M [<not-advertise|cost (0-16777215)>]

Example:
  router ospf
   router-id 1.1.1.1
   area 1 nssa
   area 1 nssa range 172.16.0.0/16
   area 1 nssa range 10.1.0.0/16
  !

Since regular area ranges and NSSA ranges have a lot in common,
this commit reuses the existing infrastructure for area ranges as
much as possible to avoid code duplication.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>