Split md5_crypt_verify() into three functions:
* get_role_password() to fetch user's password from pg_authid, and check
  its expiration.
* md5_crypt_verify() to check an MD5 authentication challenge
* plain_crypt_verify() to check a plaintext password.

get_role_password() will be needed as a separate function by the upcoming
SCRAM authentication patch set. Most of the remaining functionality in
md5_crypt_verify() was different for MD5 and plaintext authentication, so
split that for readability.

While we're at it, simplify the *_crypt_verify functions by using
stack-allocated buffers to hold the temporary MD5 hashes, instead of
pallocing.

Reviewed by Michael Paquier.

Discussion: https://www.postgresql.org/message-id/3029e460-d47c-710e-507e-d8ba759d7cbb@iki.fi