fsck: print "Checking filesystem..."

From #779 / @jroovy

> When using `-fsck`, the command line output looks like this:
>
> ```
> $ gocryptfs -fsck ENCRYPTED_DIRECTORY
> Password:
> Decrypting master key
> ```
>
> However, the user might think it's stuck at decrypting the master
> key. Adding extra text showing that fsck is working would be nice,
> something like:
>
> ```
> $ gocryptfs -fsck ENCRYPTED_DIRECTORY
> Password:
> Decrypting master key
> Checking filesystem...
> ```

Fixes #779

fsck.go

@@ -305,6 +305,7 @@ func fsck(args *argContainer) (exitcode int) {
 		}
 	}()
 	// Recursively check the root dir
+	tlog.Info.Println(tlog.ColorGreen + "Checking filesystem..." + tlog.ColorReset)
 	ck.dir("")
 	// Report results
 	wipeKeys()