Fall back to system-wide epigram directory.

Fall back to system-wide epigram directory.

If the per-user directory can not be opened, we currently print an
error message and exit. As pointed out by Adam Borowski, this is not
especially user-friendly.

This commit teaches tfortune to look in a second, system-wide location
in this case.  This location can be controlled with the --datadir
option of configure. The man page is updated accordingly.