Add an ability to set dumpdev via loader(8) tunable.

MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272746 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Revert r156046. We support setting dumpdev via loader tunable again.
Also change default disk name to ada.

MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272747 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f