Implement new user-space commands to create and verify EC parity data:

lfs ec resync — generates/regenerrates parity data for an existing EC file & writes it to EC components

lfs ec verify — reads back and validates parity data against the source data.

Both commands will use the designated EC read/write functionality introduced in the previous ticket to perform direct I/O to the parity components.

We need to consider the relationship between erasure coding and mirrors here.  Probably, the resync operation can annotate which components it actually resync'ed and that should take care of it