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: format tests in a common style

The lint tests do not focus on the whitespace since that is the most
boring part of code style. Therefore, format the tests to be readable
by following share/misc/style as close as possible.

lint: add common header for all tests

For those tests that didn't use GCC-style line markers such as "# 2",
the line numbers of the diagnostics stay the same. This is purely
conincidental. Before, the 3 lines came from lint's built-in
definitions (see 'builtins' in main1.c), and line number counting
continued as if nothing had happened, making the first line of the
actual file line 4. These 3 built-in lines are now replaced with 3
lines of file header.

new test.