sd-device: check netlink netns matches host netns before using monitor

Tracking down #15931 confused the hell out of me, since running homed in
gdb from the command line worked fine, but doing so as a service failed.
Let's make this more debuggable and check if we live in the host netns
when allocating a new udev monitor.

This is just debug stuff, so that if things don't work, a quick debug
run will reveal what is going on.

That said, while we are at it, also fix unexpected closing of passed in
fd when failing.