Fix Sorting Order For Parking Lots Stored In Static Realtime

When retrieving the parking lots from a MySQL database table, the current order
is "filename, cat_metric desc, var_metric asc, category".  If there are multiple
parking lots with the same cat_metric but different categories, everything is
being sorted on cat_metric first resulting in errors when loading the parking
lots.

This patch fixes the problem by sorting on the category field first, then the
cat_metric field.

(closes issue ASTERISK-21035)
Reported by: Alex Epshteyn
Patches:
  asterisk-21035-orderby.diff Michael L. Young (license 5026)
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Merged revisions 382942 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 382943 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382954 65c4cc65-6c06-0410-ace0-fbb531ad65f3