fix Mac detection

has('unix') returns 1 in MacVim because Mac uses Unix as base.
So you should check has('mac') for s:unix_p.
Additionally, to our annoyance, has('mac') returns 0 in Vim built in
Mac.  So, checking Mac should has more priority than checking Unix.
Merge pull request #5 from rhysd/fix/detect_mac

Fix Mac detection