Add a new service option, `rlimit` that allows a given rlimit to be
set for a specific service instead of globally.
Use the same parsing, now allowing text such as 'cpu' or 'rtprio'
instead of relying on the enum value for the `setrlimit` builtin
command as well.
Bug: 63882119
Bug: 64894637
Test: boot bullhead, run a test app that attempts to set its rtprio to
95, see that the priority set fails normally but passes when
`rlimit rtprio 99 99` is used as its service option.
See that this fails when `rlimit rtprio 50 50` is used as well.
Test: new unit tests
Change-Id: I4a13ca20e8529937d8b4bc11718ffaaf77523a52
