geli init: Allow initialization of multiple geli providers at once if
they use same passphrase and keyfiles.

Unique salt will be randomly generated for each provider to ensure the
Master Key for each is unique.

This change follows on from r335673 and r336602, which allowed multiple
providers to be attached in a single command.

Reviewed by: asomers
Approved by: sobomax
Differential Revision: https://reviews.freebsd.org/D16115