When pnfsdscopymr(8) is used to create a mirror of a file on a mirrored
pNFS service, it expects to find an entry in the extended attribute for
IP address 0.0.0.0.
This patch adds a "-m" option which can be used to create these entrie(s).
It also tightens up the checks for use of incompatible command line options.
This command allows a sysadmin to display or modify the pnfsd.dsfile extended
attribute used by the pNFS MDS server in various ways.
Its main use is to set a DS's IP address to 0.0.0.0 when that DS has failed,
so that it will not be used for the file when brought back online after
being repaired.
