MFC r273489 (cperciva):

MFC r273489 (cperciva):
 Populate the GELI passphrase cache with the kern.geom.eli.passphrase
 variable (if any) provided in the boot environment.  Unset it from
 the kernel environment after doing this, so that the passphrase is
 no longer present in kernel memory once we enter userland.

 This will make it possible to provide a GELI passphrase via the boot
 loader.

PR:		200448
Approved by:	re (kib)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@285249 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f