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.

 Note: head and stable/10 differ as a result of r273174, which renames
 the getenv(), setenv(), and unsetenv() functions with kern_getenv(),
 kern_setenv(), and kern_unsetenv(), which was reverted in the relevant
 parts of this change in 10-STABLE.

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@285263 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f