Give formats/filters way to know data amount requested from archive_read_data().
Update RAR test case to expect correct results from calls to archive_read_data().
Fix build failure when doing normal builds.
These are builds done running a standard './configure && make'. This
will also fix a corner case where the whence is set to anything other than
SEEK_SET, SEEK_CUR, or SEEK_END. The behavior is to use SEEK_SET by default.