lint: reword messages that apply to traditional C

branches: 1.7.2;
tests/lint: sort multiple diagnostics per line chronologically

For now, the chronologic order is not enforced but has to be established
manually, for example by removing all 'expect' comment lines and
regenerating them with 'accept.sh -u'.

While here, clean up a few instances that came up when regenerating the
'expect' comments, such as wrong indentation or needless deviation from
the 'expect+1' form.

lint: only skip 'unused' warnings after errors, not other warnings

Previously, in -w mode, any warning suppressed further 'unused'
warnings, even though there was no need to do that. This can be seen in
the test gcc_attribute_var.c, where only the last unused variable from a
function was marked as unused, the others slipped through.

Fixed by counting the errors and the warnings separately and only
combining them if actually desired.

lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

tests/lint: make expectation lines in the tests more detailed

This commit migrates msg_100 until msg_199.

tests/lint: add test for struct initialization in traditional C

lint: force each test to declare the expected diagnostics

By listing the expected diagnostics directly at the code that triggers
the diagnostics, it is easier to cross-check whether the diagnostics
make sense.

No functional change to lint itself.

lint: add a test for each message produced by lint1

Having a test for each message ensures that upcoming refactorings don't
break the basic functionality. Adding the tests will also discover
previously unknown bugs in lint.

The tests ensure that every lint message can actually be triggered, and
they demonstrate how to do so. Having a separate file for each test
leaves enough space for documenting historical anecdotes, rationale or
edge cases, keeping them away from the source code.

The interesting details of this commit are in Makefile and
t_integration.sh. All other files are just auto-generated.

When running the tests as part of ATF, they are packed together as a
single test case. Conceptually, it would have been better to have each
test as a separate test case, but ATF quickly becomes very slow as soon
as a test program defines too many test cases, and 50 is already too
many. The time complexity is O(n^2), not O(n) as one would expect.
It's the same problem as in tests/usr.bin/make, which has over 300 test
cases as well.