MFC r308137, r316312, r332361

r308137:
Fix alignment issues on MIPS: align the pointers properly.

All the 5520 GEOM_ELI tests passed successfully on MIPS64EB.

r316312:
sys/geom/eli: Switch bzero() to explicit_bzero() for sensitive data

In GELI, anywhere we are zeroing out possibly sensitive data, like
the metadata struct, the metadata sector (both contain the encrypted
master key), the user key, or the master key, use explicit_bzero.

Didn't touch the bzero() used to initialize structs.

r332361:
Introduce dry run option for attaching the device.
This will allow us to verify if passphrase and key is valid without
decrypting whole device.