geli: Allow disabling passphrase prompt

[oshogbo]

Summary

When using mixed decryption methods, it might be useful to disable
the passphrase prompt to allow the boot process to fail gracefully.
For example, if a device is not essential for booting, it can be
ignored during the boot process. Later, we can remotely access
the machine and decrypt the device, instead of requiring a user
to connect a keyboard to the server.

I have decided to implement this as a bootloader setting rather
than a geli flag. This approach allows us to simply unset this
flag during boot in the bootloader CLI, without the need for an
additional FreeBSD box to unset the geli flag on the disk.

This change is based on: https://reviews.freebsd.org/D45250

[oshogbo]

Ommit style changes.

[markj]

stand/libsa/geli/geliboot.c

39

Unrelated style changes.

sys/geom/eli/g_eli.c

1259

I suspect that some valid provider names contain characters that cannot be in a kenv variable name. Do we need to handle that somehow?

1262

Just a suggestion, plain "Skip" is too unclear IMO.