MEDIUM: log: relax some checks and emit diag warnings instead in lf_expr_postcheck()

MEDIUM: log: relax some checks and emit diag warnings instead in lf_expr_postcheck()

With 7a21c3a ("MAJOR: log: implement proper postparsing for logformat
expressions") which finally made postparsing checks reliable, we started
to get report from users that couldn't start haproxy 3.0 with configs that
used to work in the past. The current situation is described in GH #2642.

While the checks are mostly relevant, it turns out there are not strictly
needed anymore from a technical point of view. Most of them were useful in
early logformat implementation to prevent runtime bugs due to the use of
an alias or fetch at runtime from an incompatible proxy. It's been a few
versions already that the code handling fetches and log aliases is robust
enough to support fetches/aliases used from the wrong context: all it
does is that the fetch/alias will silently fail if it's not available.

This can be proved by the fact that even if the postparsing checks were
partially broken in the past, it didn't cause runtime issues (at least
on recent haproxy versions).

Most of these checks can now be seen as configuration hints: when a check
triggers, it will indicate a configuration inconsistency in most cases,
but they are some corner cases where it is not possible to know at config
time if the conditions will be met for the alias/fetch to work properly..
so instead of failing with a hard error like we did so far, let's just be
more permissive and report our findings using "diag_warning": such
warnings are only emitted when haproxy is started with '-dD' cli option.

We also took this opportunity to improve messages clarity and make them
more precise (report the offending item instead of complaining about the
whole expression because of a single element).

With this patch, configs that used to start before 7a21c3a shouldn't
trigger hard errors anymore.

This may be backported in 3.0.

(cherry picked from commit 41ca89bc6fe96a660fea992643dbc2c0844a609e)
[ada: ctx adjt]
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>