futility: export options for the sign and show commands

This declares the options for the sign and show commands in a
header file.

We want to split the code for logically separate file types into
separate source files, but we don't want to have multiple
option-parsing routines, so that we can be sure we're using the
same option names consistently (for example, --hash_alg always
takes the same args and means the same thing).

BUG=chromium:231574
BRANCH=none
TEST=make runtests

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Change-Id: I939bd19ba199b4c44eb41cff3571cff88df9a181
Reviewed-on: https://chromium-review.googlesource.com/262896
Reviewed-by: Randall Spangler <rspangler@chromium.org>