Cache GELI passphrases entered at the console during the boot process,
in order to improve user-friendliness when a system has multiple disks
encrypted using the same passphrase.

When examining a new GELI provider, the most recently used passphrase
will be attempted before prompting for a passphrase; and whenever a
passphrase is entered, it is cached for later reference.  When the root
disk is mounted, the cached passphrase is zeroed (triggered by the
"mountroot" event), in order to minimize the possibility of leakage
of passphrases.  (After root is mounted, the "taste and prompt for
passphrases on the console" code path is disabled, so there is no
potential for a passphrase to be stored after the zeroing takes place.)

This behaviour can be disabled by setting kern.geom.eli.boot_passcache=0.

Reviewed by:	pjd, dteske, allanjude
MFC after:	7 days