lint: fix computation of bit-field width

When bit-fields in packed structs were added on 2009-10-02, lint assumed
that they would only use 'signed int' or 'unsigned int' as storage unit,
even though C99 also allows _Bool.

The cleanup commit for decl.c 1.225 from 2021-08-28 accidentally changed
the rounding mode for bit-field storage units from round-up to
round-down.

lint: add quotes around placeholders for the remaining messages

Reword some of the messages slightly, exchanging brevity for clarity.

Message 138 is kept as-is, as it is not yet covered by any tests.

Message 240 is kep as-is, as it is unreachable.

tests/lint: add tests for empty and incomplete structs

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.