Also add a define NM_PLATFORM_ACTION_KIND_SIMPLE. It makes the
uses of "simple" grepable.

It only makes sense to call delete() with NMPObjects that
we obtained from the platform cache. Otherwise, if we didn't
get it from the cache in the first place, we wouldn't know
what to delete.

Hence, the input argument is (almost) always an NMPObject
in the first place. That is different from add(), where
we might create a new specific NMPlatform* instance on the
stack. For add() it makes slightly more sense to have different
functions depending on the type. For delete(), it doesn't.

The "onlink" flag for IPv4 routes is part of the route ID.
Consider it in nm_platform_ip4_route_cmp().

Also, allow configuring the flag when adding a route.

Note that for IPv6, the onlink flag is still ignored.
Pretty much like kernel does.

A cast macro, that does some static type checking (of the pointer).

Setting the MTU failes under regular conditions, for example when
setting the MTU of a master larger then the MTU of the slaves.

Logging a warning it too alarming.