Speed up pg_checksums in cases where checksum already set

The attached patch makes an optimization to pg_checksums which prevents
rewriting the block if the checksum is already what we expect. This can
lead to much faster runs in cases where it is already set (e.g. enabled ->
disabled -> enable, external helper process, interrupted runs, future
parallel processes). There is also an effort to not sync the data directory
if no changes were written. Finally, added a bit more output on how many
files were actually changed, e.g.:

Checksum operation completed
Files scanned:   1236
Blocks scanned:  23283
Files modified:  38
Blocks modified: 19194
pg_checksums: syncing data directory
pg_checksums: updating control file
Checksums enabled in cluster

Cheers,
Greg