lint: in C23 mode, do not warn about an empty parameter list

lint: s/illegal/invalid/g

Lint does not provide legal advice.

lint: remove no-op variant of the unary '&' operator

Neither K&R 1978 nor any of the C standards mentions that &function or
&array would be a no-op. This no-op behavior produced wrong warnings in
sbin/gpt.

tests/lint: add a few more tests to increase code coverage

branches: 1.7.2;
tests/lint: show that in C23 mode, bool is not yet defined

The tests in C11 and C23 mode look the same right now but will change
soon.

The warnings in the C11 test disappeared because after the first error,
warnings about unused variables are suppressed by check_variable_usage,
as they are often wrong.

tests/lint: sort multiple diagnostics per line chronologically

For now, the chronologic order is not enforced but has to be established
manually, for example by removing all 'expect' comment lines and
regenerating them with 'accept.sh -u'.

While here, clean up a few instances that came up when regenerating the
'expect' comments, such as wrong indentation or needless deviation from
the 'expect+1' form.

lint: make '_Static_assert' only available in C11 or C23 mode

tests/lint: test declarations in C11 and C23

lint: in C23 mode, recognize the keyword 'thread_local'

lint: reject _Noreturn if it occurs in invalid places

C11 introduced _Noreturn as a function-specifier, not as a type
attribute. The latter may occur in more places.

tests/lint: add test for _Noreturn from C11