rctl(4): support throttling resource usage to 0

[jah]

Summary

For rate-based resources that support throttling (e.g.
readiops/writeips), this fixes a divide-by-zero panic when rctl(8)
passes 0 as the throttle value. For these resources, treat
zero-throttle requests as requests to suspend forward progress as long
as possible using the duration specified in
kern.racct.rctl.throttle_max.

PR: 251803
Reported by: [e-mail]
MFC after: 1 week