stand/lua: Add optional GELI passphrase prompt

Prompt for GELI passphrase when geom_eli_passphrase_prompt has been set to
"YES" in loader.conf(5).

This entailed breaking out the password prompt into its own function that
can be reused between the password compare bits and this prompt that simply
takes the entered password and passes it along in the environment as
kern.geom.eli.passphrase.

I've also added a TODO to re-evaluate later if we want the "password
masking" -- it is currently not functional, so one still can't observe the
length of the password typed at the prompt.