Two new routines to allocate a hash context and to free it are created,
as these become necessary for the goal behind this refactoring: switch
the all cryptohash implementations for OpenSSL to use EVP (for FIPS and
also because upstream does not recommend the use of low-level cryptohash
functions for 20 years).  Note that OpenSSL hides the internals of
cryptohash contexts since 1.1.0, so it is necessary to leave the
allocation to OpenSSL itself, explaining the need for those two new
routines.  This part is going to require more work to properly track
hash contexts with resource owners, but this not introduced here.
Still, this refactoring makes the move possible.

This reduces the number of routines for all SHA2 implementations from
twelve (SHA{224,256,386,512} with init, update and final calls) to five
(create, free, init, update and final calls) by incorporating the hash
type directly into the hash context data.

The new cryptohash routines are moved to a new file, called cryptohash.c
for the fallback implementations, with SHA2 specifics becoming a part
internal to src/common/.  OpenSSL specifics are part of
cryptohash_openssl.c.  This infrastructure is usable for more hash
types, like MD5 or HMAC.

Any code paths using the internal SHA2 routines are adapted to report
correctly errors, which are most of the changes of this commit.  The
zones mostly impacted are checksum manifests, libpq and SCRAM.

Note that e21cbb4b was a first attempt to switch SHA2 to EVP, but it
lacked the refactoring needed for libpq, as done here.

This patch has been tested on Linux and Windows, with and without
OpenSSL, and down to 1.0.1, the oldest version supported on HEAD.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200924025314.GE7405@paquier.xyz