chan_sip: Add support for setting DTLS configuration in the general section.

Configuration of DTLS in the general section will be applied to any users
or peers. If configuration exists at their level it overrides the general
section values.

ASTERISK-24128 #close
Reported by: Michael K.
patches:
  dtls_default_settings.patch submitted by Michael K. (license 6621)

Review: https://reviewboard.asterisk.org/r/3867/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@427950 f38db490-d61c-443f-a65b-d21fe96a405b