loader.efi: replace HandleProtocol() with OpenProtocol()

The HandleProtocol() is deprecated interface and we should use OpenProtocol()
instead. Moreover, in some firmware implementation(s), the HandleProtocol()
does return device path using static storage, so we can not keep the value
returned there. With same firmware, the OpenProtocol() does return data we
do not need to clone.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D21162