Avoid leaking data from the kernel environment: When we convert the
initial static environment to a dynamic one, zero the static environment
buffer, and zero individual values when kern_unsetenv and freeenv are
called.

Tested by:	kmoore (VM memory dump + grep)
Tested by:	cperciva (kernel panic dump + grep)
Fix some buglets in the error-handling of getdevice(). In particular, report
an error if the argument to pciconf -a doesn't have a unit number, rather
than triggering an assertion failure.

PR:		194506
Reported by:	Anthony Cornehl <accornehl@gmail.com>
Sponsored by:	EMC / Isilon Storage Division
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