vyos-netlinkd: T9086: drain netlink during commit to avoid stale link-DOWN

Original bug: while a commit held the lock, the daemon called sleep(1)
without reading the netlink socket. The pending message stayed queued
in the kernel buffer, so once the commit ended, a stale event (e.g. a
DOWN from a disable that had already been re-enabled) got processed as
if it were fresh - stopping dhclient on an interface that was actually
back UP.

The applied fix keeps only the two pieces that are actually
load-bearing:

1. Drain instead of sleep - while commit_in_progress2() is true, still
   call ipr.get() to read and discard any pending netlink messages, so
   nothing stale survives to be processed after the commit ends.
2. Reconcile on commit end - once the commit finishes, walk all
   matching interfaces, compare live sysfs operstate against the last
   state the daemon acted on (_iface_prev_operstate), and run the
   normal DHCP handler for any that disagree. This catches a real
   transition that happened only inside the drain window and would
   otherwise be silently lost.

Dropped from the original fix: an unconditional live-sysfs override
inside _handle_dhcp_events() that ran on every event, not just around
commits - redundant with reconcile, and risked discarding genuine fast
link flaps unrelated to any commit.

Co-authored-by: Christian Breunig <christian@breunig.cc>

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