set properly in the struct buf with vinum:
Fix locations where B_READ was cleared in the old code but
b.b_iocmd wasn't set to BIO_WRITE
Fix propogation of b_iocmd
Correct comments to reflect reality
Don't compare b_flags with BIO_READ, it's in b_iocmd.
Submitted by: Bernd Walter <ticso@cicely.de>