init: support setting rlimits per service

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