ip_mroute: refactor bw_meter API

[wma]

Summary

API should work as following:
- periodicaly report Lower-or-EQual bandwidth (LEQ) connections
  over kernel socket, if user application registered for such
  per-flow notifications
- report Grater-or-EQual (GEQ) bandwidth as soon as it reaches
  specified value in configured time window

Custom implementation of callouts was removed. There is no
point of doing calout-wheel here as generic callouts are
doing exactly the same. The performance is not critical
for such reporting, so the biggest concern should be
to have a code which can be easily maintained.

This is a preparation for complete locking rework here 
which is now highly inefficient.

[mw]

sys/netinet/ip_mroute.c

1893

Wouldn't callout_drain be a safer option here?

2016

The formatting looks broken in the above block.

2046

Unrelated change, please drop this addition.

sys/netinet/ip_mroute.h

289

s/Grater/Greater/

[mw]

sys/netinet/ip_mroute.c

1768

Please move || operator to the previous line.

1825

no need to break line for == flags. Also, please move operators to the preceding line when line-breaking.