genhash: generalize the hash algo parts, add SHA1

This patch adds support for hash algo suite extension with SHA1 being
a first one to be available together with a default MD5.

The remaining change on the health-checker subsystem side is to make
analogous modifications and to teach it to recognize the intended
hash algorithm based on the length of the digest (provided that extra
care is taken that no two algorithms will ever alias in this regard).

Also the test script for genhash was extended to conditionally use SHA1.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>