Incremental: accept '--no-degraded' as a deprecated option

Commit 3288b419 (Revert "Incremental: honor --no-degraded to delay assembly")
killed the --no-degraded flag since commit 97b4d0e9 (Incremental: honor
an 'enough' flag from external handlers) made this the default behavior
of -I, and brought -I usage for external/container formats in line with
native metadata.  However, this breaks existing usages of '-I
--no-degraded', so allow it as a deprecated option.

Starting a degraded container, like the native metadata case, requires -R.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Ignacy Kasperowicz <ignacy.kasperowicz@intel.com>

mdadm.c

@@ -691,6 +691,8 @@ int main(int argc, char *argv[])
 		"     'summaries', 'homehost', 'byteorder', 'devicesize'.\n");
 			exit(outf == stdout ? 0 : 2);
 
+		case O(INCREMENTAL,NoDegraded):
+			fprintf(stderr, Name ": --no-degraded is deprecated in Incremental mode\n");
 		case O(ASSEMBLE,NoDegraded): /* --no-degraded */
 			runstop = -1; /* --stop isn't allowed for --assemble,
 				       * so we overload slightly */