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/log_test.go

@@ -39,7 +39,7 @@ func TestLogLevelString(t *testing.T) {
 	}{
 		{LogLevelEmergency, "EMERGENCY"},
 		{LogLevelDebug, "DEBUG"},
-		{99, "LogLevel(99)"},
+		{99, "Unknown(99)"},
 	} {
 		logLevel := test.in
 		got := logLevel.String()