Make EVP_Digest*() routines return a value.

TODO: update docs, and make soe other routines
which use EVP_Digest*() check return codes.

eliminate some of the -Wcast-qual warnings (debug-ben-strict target)

now and I'm putting it back to 'make test' later today.

Submitted by:
Reviewed by:
PR:

1. The already released version was 0.9.1c and not 0.9.1b

2. The next release should be 0.9.2 and not 0.9.1d, because
   first the changes are already too large, second we should avoid any more
   0.9.1x confusions and third, the Apache version semantics of
   VERSION.REVISION.PATCHLEVEL for the version string is reasonable (and here
   .2 is already just a patchlevel and not major change).
tVS: ----------------------------------------------------------------------