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

Just the most rudimentary tests.

We're going to need that one for TC filter & action support.

<linux/tc_act/tc_defact.h> was moved to user-space API only in 2013
by commit 5bc3db5c9ca8407f52918b6504d3b27230defedc. Our travis CI currently
fails to build due to that.

Re-implement the header.

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.