pgsql: Refactor the code for verifying user's password.

Refactor the code for verifying user's password.

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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e7f051b8f9a6341f6d3bf80b29c1dbc1837be9ab

Modified Files
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src/backend/libpq/auth.c | 18 +++-
src/backend/libpq/crypt.c | 217 ++++++++++++++++++++++++++++------------------
src/include/libpq/crypt.h | 9 +-
3 files changed, 153 insertions(+), 91 deletions(-)