Calculating file checksums aint legacy activity

Calculating file checksums aint legacy activity
- move the checksumming into librpmio
- rename domd5() to rpmDoDigest() with future expansion for specifying
  digest algorithm type (ignored for now)
- rearrange parameters to leave return types last
- one less rpmio_internal outside rpmio/, whee...
- used all over the rpm tree, and it knows about handling prelinked files
  so potentially useful for outside users, might as well export it
- new public header rpmfilutil.h for collecting this sort of stuff