Allow access_group to be specified as a name.

The access_group flag previously had to be a GID. This change allows a
name to be specified that will be converted into a GID before use. It
also adds checks to ensure that this conversion succeeds.

BUG=chromium-os:33830
TEST=manual

Change-Id: Iecdc30d4fee517d48d23ead6451b2ec9280b9d81
3 files changed