drbg: revert renamings of the generate and reseed counter

drbg: revert renamings of the generate and reseed counter

The original names were more intuitive: the generate_counter counts the
number of generate requests, and the reseed_counter counts the number
of reseedings (of the principal DRBG).

    reseed_gen_counter  -> generate_counter
    reseed_prop_counter -> reseed_counter

This is the anologue to commit 8380f453ec81 on the 1.1.1 stable branch.
The only difference is that the second renaming has already been reverted
on the master branch.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12941)