util: add Mac-specific check whether we're in a GUI session (issue2553) The previous test assumed that 'os.name' was "mac" on Mac OS X. This is not the case; 'mac' was classic Mac OS, whereas Mac OS X has 'os.name' be 'posix'. Please note that this change will break Mercurial on hypothetical non-Mac OS X deployments of Darwin. Credit to Brodie Rao for thinking of CGSessionCopyCurrentDictionary() and Kevin Bullock for testing.