lint: clean up the wording of a few diagnostics

lint: warn about extern declarations outside headers

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

tests/lint: replace 'expect' comments with 'expect+-' comments

The 'expect+-' comments provide more context, which makes it easier to
read the .c files on their own, without having to look up the ac

tests/lint: replace 'expect' comments with 'expect+-' comments

The 'expect+-' comments provide more context, which makes it easier to
read the .c files on their own, without having to look up the actual
diagnostics in the .exp files.

Add tests for messages 105 and 106, which were about the obscure feature
of some traditional C compilers that allowed the expression 'x->member'
to access a struct member, even if 'x' had integer type.

The remaining tests will be migrated in a future commit.

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lint: add expections to tests

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

lint: add tests for some messages

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
previo

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.

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