rman: Add rman_get/set_type

This permits associating a resource type (e.g. SYS_RES_MEMORY) with a
struct resource.

I considered adding a new field to struct rman to store the type and
only providing rman_get_type as an accessor. However, changing
'struct rman' is an ABI breakage. I might revisit this in main, but
the current approach is MFC'able.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44122