Populate the GELI passphrase cache with the kern.geom.eli.passphrase

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; FreeBSD's loader does not yet do this, but GRUB (and PCBSD)
will have support for this soon.

Tested by:	kmoore