src/libpcp_pmda/src/open.c: unix domain socket setup changes
src/libpcp_pmda/src/open.c: unix domain socket setup changes

Logic here was wrong but oftenly benignly so.  Unfortunately on
Mac OS X our luck ran out ... if getpwnam() succeeds, then errno
(aka oserror()) is undefined.

Fixed a number of failing QA tests where the PMDA was using a
Unix domain socket for IPC to pmcd.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp