xfs_db: improve argument naming in set_cur and set_iocur_type

xfs_db: improve argument naming in set_cur and set_iocur_type

In set_cur and set_iocur_type, the current naming for arguments
type, block number, and length are t, d, and c, respectively.
Replace these with more intuitive and descriptive names:
type, blknum, and len. Fix type of blknum (xfs_daddr_t) to be
consistent with that of libxfs_readbuf where it's used.
Additionally remove extra blank line in io.c.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>