Allocate GID to use reserved disk space.

Filesystems allow the setting of the "resgid" parameter to designate
a GID that is allowed to use the "reserved" disk space (in addition
to UID 0).  We'll be granting this GID to critical system processes,
so that the system is usable enough for the user to free up disk
space used by abusive apps.

Test: builds, boots
Bug: 62024591
Change-Id: I2d166f3b730f0a3e7279fb40f12db7413c1dadad