Keep an open file handle on the local file currently open: this way, we can get it back from it, in case for some reason (read: Firefox blindly removing temporary files) it gets "deleted".

Of course, this works (and thus it is activated) only on UNIX systems (as the file is not deleted for real until there are open handles on it).

(If not wanted, this behavior can be disabled by export'ing OKULAR_NO_KEEP_FILE_OPEN to 1.)

Also, in case the local file gets deleted but the real document is remote, use its (remote) URL for the copy.