lint: warn about extern declarations outside headers

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

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.

tests/lint: fix typos in comment in expr_range

branches: 1.2.2;
lint: warn about unreachable case labels for '&&'

See octeon_gmxreg.h 1.2 from 2020-06-18 for an example, where
RXN_RX_INBND_SPEED was cleaned up without adjusting the corresponding
code in octeon_gmx.c.

tests/lint: test bitwise mismatch in switch statement

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.

tests/lint: fix typos in comment in expr_range

branches: 1.2.2;
lint: warn about unreachable case labels for '&&'

See octeon_gmxreg.h 1.2 from 2020-06-18 for an example, where
RXN_RX_INBND_SPEED was cleaned up without adjusting the corresponding
code in octeon_gmx.c.

tests/lint: test bitwise mismatch in switch statement