PJSIP: Force transport on contact rewrite

If contact rewriting is enabled but the contact differs in transport
from what is actually being used, messages after the initial INVITE
transaction can be sent to an incorrect transport/port combination. In
the case where this bug occurred the remote party never received a BYE
since it was sent to the remote party's TCP port over UDP.

Review: https://reviewboard.asterisk.org/r/4032/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@424244 f38db490-d61c-443f-a65b-d21fe96a405b
PJSIP: Handle defaults properly

This updates the code behind PJSIP configuration options with custom
handlers to deal with the assigned default values properly where it
makes sense and adjusting the default value where it doesn't. Before
applying this patch, there were several cases where the default value
for an option would prevent that config section from loading properly.

Reported by: Thomas Thompson
Review: https://reviewboard.asterisk.org/r/4019/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@424263 f38db490-d61c-443f-a65b-d21fe96a405b