z/OS: Updated to build libzmq git master

These patches make it possible to build and run libzmq git master (4.1.x-development) on z/OS UNIX System Services by:

0.  Transferring source from git to z/OS UNIX System Services platform (instructions in builds/zos/README.md)

0.  builds/zos/makelibzmq

0.  builds/zos/maketests

0.  builds/zos/runtests

Tests to run are now auto-detected by parsing tests/Makefile.am in shell script (pretty rough'n'ready); tests known to fail are skipped including all TIPC tests as these appear to be Linux specific.   All but one of the remaining tests pass (test\_proxy currently hangs, and times out; unsure why.)

Includes additional build/portability fixes found while trying to build:

*   src/metadata.hpp: key of map is implicitly const; explicitly making it const is non-portable

*   tests/*\_tipc.cpp: include testutil.hpp when using SETTLE\_TIME.

Plus one possible-ZOS-specific change (src/thread.cpp) to avoid calling pthread_{get,set}schedparam() on z/OS.  (In practice I think this applies to some other platforms so may require some more portability attention.)
Why did you drop the const qualifier here?