lint: since C99, a non-void function must return a value

lint: distinguish between arguments and parameters

lint: clean up wording in diagnostics

Use the term 'parameter' as defined in C99 3.15.

lint: warn about extern declarations outside headers

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

tests/lint: add more details to messages in msg_200 until msg_299

Add some tests that were previously empty. Some other tests are still
empty.

tests/lint: one comment per expected diagnostic

This makes it possible to check for diagnostics that contain commas.

lint: add expections to tests

msg_098: fix suffix for floating point constant
msg_127: remove prototype
msg_146: fix return type

lint: rename build_ampersand to build_address

At that stage of analysis, the ampersand is no longer ambiguous, it has
already been resolved as the address-of operator, instead of the
bitwise-and operator.

lint: fix function as controlling expression (since 2020-12-31)

It's perfectly valid to directly use a function name as the controlling
expression of an if statement. That function name is converted
implicitly to a pointer to that function, and that is a scalar value
then.

Spotted by christos in lib/libpthread/pthread.c:634.

lint: demonstrate wrong message 204 (since 2020-12-31)

In func.c 1.39 from 2020-12-31 18:51:28, the check that controlling
expressions are indeed scalar was extended from while and for loops to
if statements as well. It just seemed to have been an oversight.

This revealed a bug in lint, which didn't accept the following valid
code snippet from lib/libpthread/pthread.c:634:

void _malloc_thread_cleanup(void) __weak;
...
if (_malloc_thread_cleanup)
_malloc_thread_cleanup();

Testing a function (instead of a function pointer) for truthiness is
probably rare since most functions are defined unconditionally. For
weak functions it comes in handy though.

Clang-Tidy suggests to prefix the function with '&' to silence its
warning. Doing that revealed a non-obvious behavior in build_ampersand,
which does not add the AMPER node to the expression even though it is
clearly mentioned in the code. That is left for further research.

Once the original bug is fixed, it probably doesn't matter whether the
AMPER is discarded or retained since check_controlling_expression would
add it back. There's probably a reason though to sometimes discard the
AMPER and sometimes retain it.

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.