lint: warn about function definitions without header declaration

The existing warning was only issued for function declarations, not for
function definitions.

The interesting change in the tests is in msg_351.c. Many other tests
use non-static functions due to their syntactic brevity. In these
tests, the warning is disabled individually, to allow new functions to
be added without generating warning 351.

tests/lint: merge duplicate tests for C11 _Atomic

lint: add support for C11 '_Atomic' as atomic-type-specifier

Following the C11 grammar, the keyword '_Atomic' needs to be a separate
syntactic category, to avoid further conflicts in the grammar.

The two newly added conflicts in the grammar would come into play when
mixing traditional C with C11, in a type name without an implicit 'int'.
If the type '_Atomic(int)*' were parsed as '_Atomic int(int)*', the
trailing '*' would be a syntax error.

lint: add support for the C11 type qualifier '_Atomic'

That keyword can be used as a type specifier as well, support for that
will be added later.

tests/lint: test _Atomic, added in C11