Add copies to the allowed keyword for restore.

Its seems that you can use a set of copy jobs to do a restore when you
specify the copies keyword on the restore commandline.

e.g. restore copies will select instead of the normal
full/differential/incremental jobs the copies of those Jobs. When doing
offsite SD-SD replication this can be a good selection criteria when you
lost your primay storage daemon but can restore from the secondary
or standby storage daemon. As long as you have a working database this
is a good DR procedure. You still need a working director and database
but as the database can be replicated as an HA solution and a director
also have a cold standby this could be part of a solution.

Fixes #36: Implementation of restore using a set of copies of an original job

src/dird/ua_cmds.c

@@ -170,7 +170,7 @@ static struct cmdstruct commands[] = {
      NT_(""), false },
    { NT_("restore"), restore_cmd, _("Restore files"),
      NT_("where=</path> client=<client-name> storage=<storage-name> bootstrap=<file>\n"
-         "\trestorejob=<job-name> comment=<text> jobid=<jobid> done select all"), false },
+         "\trestorejob=<job-name> comment=<text> jobid=<jobid> copies done select all"), false },
    { NT_("relabel"), relabel_cmd, _("Relabel a tape"),
      NT_("storage=<storage-name> oldvolume=<old-volume-name>\n"
          "\tvolume=<new-volume-name> pool=<pool-name> [ encrypt ]"), false },