coregrind files shall use vg_assert not tl_assert.
Tool files shall use tl_assert not vg_assert.
Fix code accordingly.
Adapted check_headers_and_includes to make sure the code
stays clean in that respect.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14542 a5019735-40e9-0310-863c-91ae7b9d1cf9