geli attach multiple providers

Allow attaching of multiple geli providers at once if they use same
passphrase and keyfiles.

This is helpful when the providers being attached are not used for boot,
and therefore the existing code to first try the cached password when
tasting the providers during boot does not apply.

Multiple providers with the same passphrase and keyfiles can be attached
at the same time during system start-up by adding the following to
rc.conf:

geli_groups="storage backup"
geli_storage_flags="-k /etc/geli/storage.keys"
geli_storage_devices="ada0 ada1"
geli_backup_flags="-j /etc/geli/backup.passfile -k /etc/geli/backup.keys"
geli_backup_devices="ada2 ada3"

Reviewed by: wblock, delphij, jilles
Approved by: sobomax (src), bcr (doc)
Differential Revision: https://reviews.freebsd.org/D12644