ERT-530: Added util_abort() testing if we tro to write to a readonly fs.
ERT-530: Added safe_cast() macro to block_fs.
ERT-530: Changed locking + read_only behaviour slightly; is use_lockfile == true you will get a lockfile even if read_only == true.
ERT-530: Removed unused field index_time from block_fs struct.
ERT-530: Minor header change.
ERT-530: Added testing of readonly and lockfile behaviour of block_fs.