Resolve duplicate label used in multiple priorities for the same extension.

Prior to this patch, if labels with the same name were used for different priorities in
the same extension, the new label would be accepted, but it would be unusable since
attempts to reach that label would just go to the first one. Now pbx.c detects this,
generates a warning in logs, and culls the label before adding it to the dialplan.

(closes issue ASTERISK-18807)
Reported by: Kenneth Shumard
Patches:
	pbx.c.patch uploaded by Kenneth Shumard (License 5077)
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Merged revisions 346954 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 346955 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@346956 65c4cc65-6c06-0410-ace0-fbb531ad65f3