Remove use of 'head' and add hlint warning for it

Since 'head' is unsafe to use in most cases, this patch removes its
use from most of the code, adds a lint warning for it (and for tail as
well), and adds override annotations in the few cases where it's
actually OK to use it (mainly when using head over the result of
functions which guarantee to return a non-empty list by documentation,
not type).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>