head: incorporate "all but last" option into Mode
head: incorporate "all but last" option into Mode

Refactor the `Mode` enum in the `head.rs` module so that it includes
not only the mode type---lines or bytes---but also whether to read the
first NUM items of that type or all but the last NUM. Before this
commit, these two pieces of information were stored separately. This
made it difficult to read the code through several function calls and
understand at a glance which strategy was being employed.

uutils/coreutils,uutils/coreutils,uutils/coreutils