This commit adds ZMTP connection heartbeats described in
http://rfc.zeromq.org/spec:37/ZMTP.

Of course people still "can" distributed the sources under the
LGPLv3. However we provide COPYING.LESSER with additional grants.

Solution: specify these grants in the header of each source file.

Solution: update for 2015

Fixes #1320

Tested on FreeBSD 10.

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

For exchanging tokens.

E.g., both client and server need to produce and process GSSAPI tokens.