Add "GELI Passphrase:" prompt to boot loader.

A new loader.conf(5) option of geom_eli_passphrase_prompt="YES" will now
allow you to enter your geli(8) root-mount credentials prior to invoking
the kernel.

See check-password.4th(8) for details.

Differential Revision: https://reviews.freebsd.org/D2105
Reviewed by: imp, kmoore
Discussed on: -current
MFC after: 3 days
X-MFC-to: stable/10
Relnotes: yes