lint: warn about extern declarations outside headers

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

tests/lint: merge tests for ':?' with null pointer constant

lint: fix the result type of '?:' for void pointers

The change from 2015-07-29 was wrong since that rule only applies to
null pointer constants but not to other void pointers.

tests/lint: demonstrate wrong type for '?:' expressions

tests/lint: make 'expect+-' comments stricter

Previously, the expectations from these comments were already satisfied
if the expectation occurred somewhere in the actual message from lint.
This mean

tests/lint: make 'expect+-' comments stricter

Previously, the expectations from these comments were already satisfied
if the expectation occurred somewhere in the actual message from lint.
This meant that the prefix 'error:' or 'warning:' could be omitted from
the 'expect' comment. These omissions were hard to see in a manual
review. Now any omissions must be visually marked with '...'.

The test msg_342 now reports its messages properly as being in the file
msg_342.c, rather than msg_341.c. This had been a copy-and-paste
mistake.

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tests/lint: expect complete messages in feature tests

Previously, the tests contained many comments like /* expect: 123 */,
which were useless to a casual reader since nobody is expected to learn
li

tests/lint: expect complete messages in feature tests

Previously, the tests contained many comments like /* expect: 123 */,
which were useless to a casual reader since nobody is expected to learn
lint's message IDs by heart. Replace these with the complete
diagnostics, to show what lint is complaining about.

The tests named msg_*.c have been left unmodified since they mention the
full message text in their header comment.

No functional change.

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tests/lint: prevent typo from tree.c 1.264 from happening again