check-code: automatically preppend "warning: " to all warning messages
Some warnings had "warning: " at the beginning of their message. Now this
is done consistent for all messages.
Especially in test-check-code-hg.t it is an advantage to see warnings at once
because only exceptions to them are tolerated. It is (almost) as obvious as
before
a6180647ea.
The prefix will not remain when a warning is changed to a failure. A change
like
a91387a37f will not be necessary anymore.