res_stir_shaken: Add "ignore_sip_date_header" config option.

res_stir_shaken: Add "ignore_sip_date_header" config option.

UserNote: A new STIR/SHAKEN verification option "ignore_sip_date_header" has
been added that when set to true, will cause the verification process to
not consider a missing or invalid SIP "Date" header to be a failure.  This
will make the IAT the sole "truth" for Date in the verification process.
The option can be set in the "verification" and "profile" sections of
stir_shaken.conf.

Also fixed a bug in the port match logic.

Resolves: #1251
Resolves: #1271
(cherry picked from commit 6b107e90f33fe168055f5a7a38f5237ddacfa599)