ccanlint: allow per-file restrictions, so we don't have to remove all tests.

ccanlint: allow per-file restrictions, so we don't have to remove all tests.

For example, valgrind can't handle some things, so we may need to
switch it off, but we don't want to do that for every test.

So, start a standard format for per-file restrictions, eg:

    Ccanlint: tests_pass_valgrind test/foo.c:FAIL

For the moment, only tests_pass_valgrind takes options, so it has to
change: we now have a helper which returns the array of options
applicable to a given file.