geli init for first time

[Robert Simmons]

After you perform "geli init" and "geli attach" you must use dd to
initialize the new provider before you run newfs.  If you had enabled
authentication of some kind during the init step, when you attach the
provider you get a series of errors such as these:
GEOM_ELI: ada0p4.eli: 4096 bytes corrupted at offset 0.
GEOM_ELI: ada0p4.eli: 4096 bytes corrupted at offset 4096.

and lots more errors exactly like this.  These are produced because
the checksums have not been initialized yet.  In the manpage of geli
it states "It is recommended to write the whole provider before the
first use, in order to make sure that all sectors and their
corresponding checksums are properly initialized into a consistent
state."

These errors are no problem at all, but they look scary when you first
encounter them and you don't realize why they are being produced.

I propose that a switch be added to "geli attach" that signifies that
it is the first time that a provider is being attached and these
errors are to be suppressed.

An alternative is to add something to the end of the DATA
AUTHENTICATION section of the geli(8) man page that mentions why these
errors occur and that they can be safely ignored.

Thoughts?