init: accept -1 or 'unlimited' for an infinite rlimit

Due to a bug with ParseUint(), init would defacto accept -1 for an
infinite rlimit, but only on 64bit devices.  That bug is now fixed,
such that -1 would be rejected by ParseUint() for all devices.

This change explicitly checks for -1 for all devices or 'unlimited' to
match ulimit's reporting and accepts either as an infinite rlimit.

Bug: 112668205
Test: new (and old) unit tests
Change-Id: Ie28ff622cdf375a65ceb5f32ffb14fb3d5d9f2ba