apply: refactor `git apply` option parsing

apply: refactor `git apply` option parsing

Parsing `git apply` options can be useful to other commands that
want to call the libified apply functionality, because this way
they can easily pass some options from their own command line to
the libified apply functionality.

This will be used by `git am` in a following patch.

To make this possible, let's refactor the `git apply` option
parsing code into a new libified apply_parse_options() function.

Doing that makes it possible to remove some functions definitions
from "apply.h" and make them static in "apply.c".

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>