another iteration on ZMQ_PROBE_ROUTER

now it properly works for ROUTER

Updated ZAP PLAIN request to follow latest draft

- username and password sent as two string frames
- fixed test case to match

- REQ and REP sockets don't have HWM issues
- ZMQ_DONTWAIT applies to DEALER and PUSH only

Add ZAP support

Packaging of probe function

- renamed to ZMQ_PROBE_ROUTER

- Cleaned up man page a little
- Wrote test case tests/test_router_probe.cpp

implement it for DEALER tocket.
+documentation

Make "./configure --with-system-pgm" detect the OpenPGM 5.2 system library.

    ./configure --with-system-pgm
detect the OpenPGM 5.2 system library.

Note that OpenPGM installs a versioned pkgconfig file
(openpgm-5.2.pc, openpgm-5.1.pc).

Fold constant expression into constant.

Fixed memory leak in stream engine

Refactoring of 'ZMQ_ROUTER_ANNOUNCE_SELF'

Rename pipe states so they are more mnemonic

implement it for DEALER tocket.
+documentation

Removed tracing on router option setting

Add ROUTER socket option to introduce self(send an empty message) to new peers, to allow ROUTER<->ROUTER auto-discovery problem.

Add ROUTER socket option to introduce self(send an empty message) to new peers, to allow ROUTER<->ROUTER auto-discovery problem.

Add ROUTER socket option to introduce self(send an empty message) to new peers, to allow ROUTER<->ROUTER auto-discovery problem.

Working on Travis builds

- made consistent with options.cpp code
- added trace print for ongoing issue with Travis CI builds

Adding support for Travis CI