checkout: split options[] array in three pieces

checkout: split options[] array in three pieces

This is a preparation step for introducing new commands that do parts
of what checkout does. There will be two new commands, one is about
switching branches, detaching HEAD... one about checking out
paths. These share the a subset of command line options. The rest of
command line options are separate.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>