apinger: improve uci and procd support

apinger: improve uci and procd support

- convert apinger into procd instances
- generate instance specific apinger.conf from uci
- hotplug handling for apinger alarms
- restart apinger interface instance on ifup action of interface
- don't exit on packet count mismatch, allows to use apinger as monitor
  for multiple targets handling
- add srcip option to target configuration, allows specifying source ip
  used to monitor target
- allow creating status file in script parseable format

Patches are ported against latest version of apinger and referenced from
https://git.pld-linux.org/?p=packages/apinger.git;a=summary

Signed-off-by: Jaymin Patel <redacted>