epair: deduplicate interface allocation code #1

Simplify epair_clone_create() and epair_clone_destroy() by
factoring out epair softc allocation / desctruction and
interface setup/teardown into separate functions.

Reviewed By: kp, zlei.huang_gmail.com
Differential Revision: https://reviews.freebsd.org/D36614
MFC after: 2 weeks