#29 detect directories without -d

[Martin Bähr]

when starting gimv, it should not be necessary to use
-d to open a directory. if an argument is a file or a
directory can be checked with stat().

needing such an argument breaks some windowmanagers
where drag+drop can be used to open an application with
an argument. the window manager usually can not be
configured to use different arguments for directories
vs. files. it will only accept one configuration for
drag+drop, and that must work for files and directories.