Sanitize XML output for PIDF bodies.

PJSIP's PIDF API does not replace angle brackets with
their appropriate counterparts for XML. So we have to
do it ourself. In this particular case, the problem had
to do with attempting to place an unsanitized SIP URI
into an XML node. Now we don't get a 488 from recipients
of our PIDF NOTIFYs.
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Merged revisions 397968 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@397969 f38db490-d61c-443f-a65b-d21fe96a405b
pbx.c: Make pbx_substitute_variables_helper_full() not mask variables.
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Merged revisions 397977 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@397978 f38db490-d61c-443f-a65b-d21fe96a405b