It appears that libzrtp would prevent a non-passive client from
enrolling with a PBX because of a broken license mode comparison.
Thanks to Ivo Kutts for the report.
FS-4223 --resolve
Thanks to Phil Zimmermann for the code and for the license exception
we needed to include it.
There remains some build system integration work to be done before
this code will build properly in the FreeSWITCH tree.