geli init with multiple providers - fix init and fix -B "none"

Apply some fixes post rS336659, which allowed multiple provders to be
initialized in a single command.

• Fix issue where second and subsequent providers would fail init. This was due to the metadata struct being zeroed after the first provider init was completed, despite containing common data required for subsequent providers.
• Fix issue where -B "none" would still result in the metadata being backed-up if multiple providers had been specified. This was due to the backupfile of "none" being incorrectly made unique for each provider by appending "-<prov>".

Tested using md(4) memory disks to confirm multiple providers can be initialized
in a single command using the same passphrase:

mdconfig -s 1m -u 0
mdconfig -s 1m -u 1
geli init -v -B none md0 md1   # set password to "test"
geli attach -v md0 md1         # enter password "test"
geli status
geli detach md0 md1
ls -lah /var/backups/md*
mdconfig -d -u 0
mdconfig -d -u 1

Expected output:

Enter new passphrase:
Reenter new passphrase:
Calculating number of iterations...
Done, using 2071395 iterations.
Metadata value stored on md0.
Metadata value stored on md1.
Done.

Enter passphrase:
Attached to md0.
Attached to md1.
Done.

      Name  Status  Components
   md0.eli  ACTIVE  md0
   md1.eli  ACTIVE  md1

ls: /var/backups/md3*: No such file or directory