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/
........

Merged revisions 424263 from http://svn.asterisk.org/svn/asterisk/branches/12
........

Merged revisions 424266 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424267 65c4cc65-6c06-0410-ace0-fbb531ad65f3