QGrpcHttp2Channel: fix maximumReceiveMessageSize(0) semantics

Amends 8c8f0e3cffa4003cd549a0a6761297f53e1a5f75.

A limit of 0 was silently expanded to the transport cap, making it
effectively unlimited. The value is now used verbatim: 0 accepts only
zero-length messages, giving callers a consistent "value is the limit"
contract and removing a hidden special case.

Also unify the environment variable handling for other env-vars that
will be introduced after this commit, and picked back manually into
older branches. This commit is the first of its kind, so we do it here
already:

 * Introduced 'macro.h2EnvFallback' to unify the environment fallbacks
 * readEnvUnsignedInt(const char *name)
 * doc section: 'Environment variable fallbacks'

Found before the QtGRPC API review.

Task-number: QTBUG-145258
Task-number: QTBUG-145399
Fixes: QTBUG-146592
Pick-to: 6.12
Change-Id: I0fc2780b6d6ddbffed2466cfe8d92d1c0324b2c5
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>