res_rtp: Fix regression when IPv6 is not available.

res_rtp: Fix regression when IPv6 is not available.

The latest Release candidate fails to create RTP streams when IPv6
is not available. Due to the changes made in September the ast_sockaddr
structure passed around to create these streams is always of AF_INET6
type, causing failure when used for IPv4. This patch adds a utility
function to check for availability of IPv6 and applies such check
at startup to determine how to create the ast_sockaddr structures.

ASTERISK-26617 #close

Change-Id: I627a4e91795e821111e1cda523f083a40d0e0c3e