Make ping-check timeout configurable in ms, thus allowing timeouts < 1s

name: Make ping-check timeout configurable in ms
about: Currently ping-check can be specified only in seconds, with a minimum value of 1s.  For some situations it might be desirable to make this even shorter.

As reported in Support ticket #14375.

The scenario is a problem with a UEFI DHCP process which appears to timeout around 600ms, due to a Bitlocker Network Unlock process. When the ping before allocate check is enabled on the DHCP server, the minimum timeout being 1 second, this results on the DHCP process on the client failing before the ping timeout is reached.

This makes using Bitlocker Network Unlock impossible to deploy on a network that is using ISC DHCP with ping-check enabled.

Is it possible to permit more granularity within the ping timeout value?

This might also benefit other environments using devices with embedded DHCP clients that don't perform their own client-based local checks for duplication before accepting a lease offer.