reject call attempts when gatekeeper is configured but not registered

(closes issue ASTERISK-21800)
Reported by: Dmitry Melekhov
Patches:
        ASTERISK-21800-1.patch
Tested by: Dmitry Melekhov
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Merged revisions 390181 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 390223 from file:///srv/subversion/repos/asterisk/branches/10


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/10-digiumphones@390227 f38db490-d61c-443f-a65b-d21fe96a405b