Refactor: libpe_status: Add pe__rsc_node flag enum

This commit adds a new pe__rsc_node flag enum containing values for
assigned, current, and pending. This indicates the criterion used to
look up a resource's location.

After a compatibility break, native_location() could use these flags
instead of an int.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>