Unwind #if spaghetti in hmac_openssl.c a bit.

Unwind #if spaghetti in hmac_openssl.c a bit.

Make this code a little less confusing by defining a separate macro
that controls whether we'll use ResourceOwner facilities to track
the existence of a pg_hmac_ctx context.

The proximate reason to touch this is that since b8bff07da, we got
"unused function" warnings if building with older OpenSSL, because
the #if guards around the ResourceOwner wrapper function definitions
were different from those around the calls of those functions.
Pulling the ResourceOwner machinations outside of the #ifdef HAVE_xxx
guards fixes that and makes the code clearer too.

Discussion: https://postgr.es/m/1394271.1712016101@sss.pgh.pa.us