init internal vars in sqlfs_proc_access based on least permission

The tricky part here is what to initialize these values to.
get_permission_data() could fail to get the values from SQLite, so then the
initialized values would be used. sqlfs_proc_access() does include checks
for whether SQLite succeeded, i.e. if (r == SQLITE_OK), but a slip in the
internal logic of this function could cause the initialized values to be
used.  Then again, sqlfs_proc_access() does not grant access, it only
checks access, so the security risk is pretty low.

fixes #1716 https://dev.guardianproject.info/issues/1716
closes #4 https://github.com/guardianproject/libsqlfs/pull/4