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.

lint: set the return type of __builtin_*_overflow to bool, not int

Needed for inetd.c in strict bool mode.

tests/lint: test GCC builtins for overflow in strict bool mode

Seen in inetd.c.