Fix For Not Overriding The Default Settings In chan_sip

The initial report was that the "nat" setting in the [general] section was not
having any effect in overriding the default setting.  Upon confirming that this
was happening and looking into what was causing this, it was discovered that
other default settings would not be overriden as well.

This patch works similar to what occurs in build_peer().  We create a temporary
ast_flags structure and using a mask, we override the default settings with
whatever is set in the [general] section.

In the bug report, the reporter who helped to test this patch noted that the
directmedia settings were being overriden properly as well as the nat settings.

(closes issue ASTERISK-21225)
Reported by: Alexandre Vezina
Tested by: Alexandre Vezina, Michael L. Young
Patches:
  asterisk-21225-handle-options-default-prob_1.8_v4.diff.diff
						Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2386/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@385008 f38db490-d61c-443f-a65b-d21fe96a405b