mwan3: use procd for locking everywhere

mwan3: use procd for locking everywhere

Replace locks on /var/run/mwan3.lock with locks via procd.

This fixes a deadlock issue where mwan3 stop would have a procd
lock, but a hotplug script would have the /var/run/mwan3.lock

Locking can be removed from mwan3rtmon since:
1) procd will have sent the KILL signal to the process during
shutdown, so it will not add routes to already removed interfaces on
mwan3 shutdown and
2) mwan3rtmon checks if an interface is active based on the
mwan3_iface_in_<IFACE> entry in iptables, and the hotplug script
always adds this before creating the route table and removes it
before deleting the route table

Fixes github issue #13704
(https://github.com/openwrt/packages/issues/13704)