Return t if the region is active and it is appropriate to act on it.
This is used by commands that act specially on the region under
Transient Mark mode.
The return value is t if Transient Mark mode is enabled and the
mark is active; furthermore, if `use-empty-active-region' is nil,
the region must not be empty. Otherwise, the return value is nil.
For some commands, it may be appropriate to ignore the value of
`use-empty-active-region'; in that case, use `region-active-p'.