pgsql: Improve some code around cryptohash functions

Improve some code around cryptohash functions

This adjusts some code related to recent changes for cryptohash
functions:
- Add a variable in md5.h to track down the size of a computed result,
moved from pgcrypto.  Note that pg_md5_hash() assumed a result of this
size already.
- Call explicit_bzero() on the hashed data when freeing the context for
fallback implementations.  For MD5, particularly, it would be annoying
to leave some non-zeroed data around.
- Clean up some code related to recent changes of uuid-ossp.  .gitignore
still included md5.c and a comment was incorrect.

Discussion: https://postgr.es/m/X9HXKTgrvJvYO7Oh@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9b584953e7bf91e342af87ef44606acd6206cd1c

Modified Files
--------------
contrib/pgcrypto/internal.c   |  4 ----
contrib/uuid-ossp/.gitignore  |  1 -
contrib/uuid-ossp/uuid-ossp.c |  4 ++--
src/common/cryptohash.c       | 20 ++++++++++++++++++++
src/common/md5_common.c       |  2 +-
src/include/common/md5.h      |  4 ++++
6 files changed, 27 insertions(+), 8 deletions(-)