Hack: Always check /etc/{passwd,shadow} in addition to defined files

Hack: Always check /etc/{passwd,shadow} in addition to defined files

When some users (eg. owner) might be in /opt/etc/passwd and some
("non-stock") are added to /opt/etc/passwd it means we have to
check both files.

Please note group is taken from /etc/group only.  Group membership
must not be managed by GUM due to the fact that assigning user to
given group requires altering file which defines the group.  Eg.
to add new user to system group 'audio' we would need to modify
/etc/group.  This breaks the requirement for rootfs to be read-only.

Change-Id: Ic63605b5f3964f166d3d5cf5332d5ee5175a7d18