dhcp: support dhcp-hostsfile

Allow IP address reservation per MAC address via dhcp-hostsfile.
After creating or changing the file send SIGHUP to the dnsmasq
process to re-read it.

The file contains information about one host per line. The format of a line is the same as text to the right of '=' in --dhcp-host.

Ref: https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html

BUG=b/321300259
TEST=manual

Change-Id: Ia81dae9fb7f286d7c82f56b815dfac8d117abab0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/moblab/+/5421108
Reviewed-by: Michal Matyjek <mmatyjek@google.com>
Commit-Queue: Tomasz Jakubowski <jakubowskit@google.com>
Tested-by: Tomasz Jakubowski <jakubowskit@google.com>