chan_sip: Support nat=auto_comedia or nat=force_rport,auto_comedia.

In the SIP channel driver chan_sip, auto_comedia was expected to be used in
tandem with auto_force_rport. Or stated differently: Only when auto_force_rport
was chosen (the default), auto_comedia worked. This change allows auto_comedia
to be set independently of the state of (auto_)force_rport. For example,
nat=force_rport,auto_comedia is useful for IPv4/IPv6 Dual Stack deployments
when IPv6 clients are behind a Firewall.

ASTERISK-26457 #close

Change-Id: Ib29d66c6dbb61648e371e01fc36c6978ddae5bc2