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.

It's going to be useful for other objects that have a type (of course)
and an ifindex.

It's not clear whether free() changes errno. Be sure about it.

https://bugzilla.gnome.org/show_bug.cgi?id=790726

We also do this for libnm, where it causes visible changes
in behavior. But if somebody would rely on the hashing implementation
for hash tables, it would be seriously flawed.

GHashTable optimizes a NULL equality function to use direct pointer
comparison. That saves the overhead of calling g_direct_equal().
This is also documented behavior for g_hash_table_new().

While at it, also don't pass g_direct_hash() but use the default
of %NULL. The behavior is the same, but consistently don't use
g_direct_hash().

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.