df: silently accept -h

Traditional Unix 'df' use -h for human readable units.
I always end up typing 'df -h' so it's frustrating to see the usage there.

src/bin/df.cpp

@@ -207,6 +207,9 @@ main(int argc, char **argv)
 					case 'b':
 						showBlocks = true;
 						break;
+					case 'h':
+						// human readable units in Unix df
+						break;
 					default:
 						ShowUsage(programName);
 				}