watchcat: fix interface reset does not work

watchcat: fix interface reset does not work

As reported in #23410 Network interface reset doesn't work as expected
on a Wireguard VPN interface and in #27927 lt2p interface won't reboot,
and mentioned in #27248, the current implementation of the option to
restart an interface when connectivity check fails for some period does
not result in an interface restart for all interface.

Notably 'virtual' interfaces such as Wireguard and L2TP do not restart.

The solution that works is to use `ifup <interface>` instead of only
changing the link status.

This commit is based on the one in #27248 by @rondoval, who unfortunately
has not updated the commit message as requested for half a year.

Signed-off-by: Daniel F. Dickinson <redacted>