fs: re-implement CutoffMode, LogLevel, TerminalColorMode with Enum

This almost 100% backwards compatible. The only difference being that
in the rc options/get output CutoffMode, LogLevel, TerminalColorMode
will be output as strings instead of integers. This is a lot more
convenient for the user. They still accept integer inputs though so
the fallout from this should be minimal.

fs/cutoffmode_test.go

@@ -22,7 +22,7 @@ func TestCutoffModeString(t *testing.T) {
 	}{
 		{CutoffModeHard, "HARD"},
 		{CutoffModeSoft, "SOFT"},
-		{99, "CutoffMode(99)"},
+		{99, "Unknown(99)"},
 	} {
 		cm := test.in
 		got := cm.String()