res_rtp_asterisk.c: Add stun_blacklist option

Added the stun_blacklist option to rtp.conf.  Some multihomed servers have
IP interfaces that cannot reach the STUN server specified by stunaddr.
Blacklist those interface subnets from trying to send a STUN packet to
find the external IP address.  Attempting to send the STUN packet
needlessly delays processing incoming and outgoing SIP INVITEs because we
will wait for a response that can never come until we give up on the
response.  Multiple subnets may be listed.

ASTERISK-26890 #close

Change-Id: I3ff4f729e787f00c3e6e670fe6435acce38be342