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.)
For reference this is what the build errors look like on z/OS with the IBM XL C/C++ compiler:

    > cat map-string.cpp
    #include <map>
    #include <string>

    int main(int argc, char *argv[]) {
      typedef std::map<const std::string, const std::string> dict;
      dict map;
    }
    > c++ -+ -Wc,xplink -Wl,xplink -o map-string map-string.cpp
    "/usr/include/xmemory", line 390.23: CCN5401 (S) The member "std::allocator<const  std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> > >::address(const_reference) const" is already declared.
    "/usr/include/xmemory", line 388.17: CCN5425 (I) "address" is defined on line 388 of "/usr/include/xmemory".
    "/usr/include/xtree", line 149.17: CCN5700 (I) The previous message was produced while processing "class std::allocator<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> > >".
    "/usr/include/xtree", line 132.15: CCN5700 (I) The previous message was produced while processing "class std::_Tree<std::_Tmap_traits<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> >,const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> >,std::less<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> > >,std::allocator<std::pair<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator...".
    "/usr/include/map", line 108.15: CCN5700 (I) The previous message was produced while processing "class std::map<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> >,const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> >,std::less<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> > >,std::allocator<std::pair<const std::_EBCDIC::_LFS_OFF::basic_string<char,std::char_traits<char>,std::allocator<char> >,const std::...".
    "./map-string.cpp", line 4.5: CCN5700 (I) The previous message was produced while processing "main(int, char *[])".
    CCN0793(I) Compilation failed for file ./map-string.cpp.  Object file not created.
    FSUM3065 The COMPILE step ended with return code 12.
    FSUM3017 Could not compile map-string.cpp. Correct the errors and try again.
    > 

Without the const on the key to std::map it compiles without errors:

    > !cat
    cat map-string.cpp
    #include <map>
    #include <string>
    
    int main(int argc, char *argv[]) {
      typedef std::map<std::string, const std::string> dict;
      dict map;
    }
    > c++ -+ -Wc,xplink -Wl,xplink -o map-string map-string.cpp
    >