pgcrypto: Remove internal padding implementation

pgcrypto: Remove internal padding implementation

Use the padding provided by OpenSSL instead of doing it ourselves.
The internal implementation was once applicable to the non-OpenSSL
code paths, but those have since been removed.  The padding algorithm
is still the same.

The OpenSSL padding implementation is stricter than the previous
internal one: Bad padding during decryption is now an error, and
encryption without padding now requires the input size to be a
multiple of the block size, otherwise it is also an error.
Previously, these cases silently proceeded, in spite of the
documentation saying otherwise.

Add some test cases about this, too.  (The test cases are in
rijndael.sql, but they apply to all encryption algorithms.)

Reviewed-by: Jacob Champion <pchampion@vmware.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/ba94c26b-0c58-c97e-7a44-f44e08b4cca2%40enterprisedb.com