qa/common.check: fixups from Mac OS X testing
qa/common.check: fixups from Mac OS X testing

1. in _check_freespace() accommodate difference in comand line
   options to df(1)
2. in _ipv6_localhost() send failure messages to stderr and return
   an empty string in the event of a failure

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