library: Update diskstat API

library: Update diskstat API

The calls for reading diskstat have been moved out of
sysinfo and into new files diskstat.[ch] These new
library calls follow the standard pattern for the
new libprocps.

vmstat is updated to use the new API and also got
the weighted IO time added.

vmstat -p previously would only show partitions, not
disks. There does not appear to be any good reason to
artifically deny a user to use this command on a disk,
rather than a partition so this restriction was lifted.

I also realised using int for devid means you can send
the library negative numbers, the index uses unsigned int.
Other similiar calls will need to be fixed too.

Signed-off-by: Craig Small <csmall@enc.com.au>