Allow disabling of 100rel support on PJSIP endpoints.

Due to an inversion error, setting 100rel=no would not actually
change the current value of the setting (which defaulted to "yes").
With this fix, the inversion is corrected.
........

Merged revisions 431420 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431436 65c4cc65-6c06-0410-ace0-fbb531ad65f3