ccanlint: check examples actually mention thing they are demonstrating.

Brad suggested this after finding one such cut & paste in str:

rusty@vivaldi:~/devel/cvs/ccan/ccan/str$ ../../tools/ccanlint/ccanlint
Example: sections demonstrate appropriate function (examples_relevant): FAIL
/home/rusty/devel/cvs/ccan/ccan/str/str.h:64:Example for strcount doesn't mention it
Total score: 37/38