pgsql: Make implementation of SASLprep compliant for ASCII characters

Make implementation of SASLprep compliant for ASCII characters

This commit makes our implementation of SASLprep() compliant with RFC
3454 (Stringprep) and RFC 4013 (SASLprep).  Originally, as introduced in
60f11b87a234, the operation considered a password made of only ASCII
characters as valid, performing an optimization for this case to skip
the internal NFKC transformation.

However, the RFCs listed above use a different definition, with the
following characters being prohibited:
- 0x00~0x1F (0~31), control characters.
- 0x7F (127, DEL).

In its SCRAM protocol, Postgres has the idea to apply a password as-is
if SASLprep() is not a success, so this change is safe on
backward-compatibility grounds:
- A libpq client with the compliant SASLprep can connect to a server
with a non-compliant SASLprep.
- A libpq client with the non-compliant SASLprep can connect to a server
with a compliant SASLprep.

This commit removes the all-ASCII optimization used in pg_saslprep() and
applies SASLprep even if a password is made only of ASCII characters,
making the operation compatible with the RFC.  All the in-core callers
of pg_saslprep() do that:
- pg_be_scram_build_secret() in auth-scram.c, when generating a
SCRAM verifier for rolpassword in the backend.
- scram_init() in fe-auth-scram.c, when starting the SASL exchange.
- pg_fe_scram_build_secret() in fe-auth-scram.c, when generating a SCRAM
verifier for the frontend with libpq, to generate it for a ALTER/CREATE
ROLE command for example.

The test module test_saslprep shows the difference this change is
leading to.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/aaEJ-El2seZHeFcG@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3d10ece612f535be15a9cb7ca31620c80db6f0e9

Modified Files
--------------
src/common/saslprep.c                              | 12 ----
.../test_saslprep/expected/test_saslprep.out       | 66 +++++++++++-----------
.../modules/test_saslprep/t/001_saslprep_ranges.pl |  5 +-
3 files changed, 34 insertions(+), 49 deletions(-)