Fix detection of free DISPLAYs

[tyilo]

The file /tmp/.X0-lock doesn't exist even though I have a display 0. However the file /tmp/.X11-unix/X0 exists instead.

[J4MMlE]

Because /tmp/.X11-unix/X_ is a socket file, testing for its existence with the -f flag (which tests for regular files) does not work. Either -S (true if file exists and is a socket) or the more generic -e (which tests only for file existence regardless of its type) should be used. Check Bash Conditional Expressions for more information.