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/terminalcolormode_test.go

@@ -17,7 +17,7 @@ func TestTerminalColorModeString(t *testing.T) {
 		{TerminalColorModeAuto, "AUTO"},
 		{TerminalColorModeAlways, "ALWAYS"},
 		{TerminalColorModeNever, "NEVER"},
-		{36, "TerminalColorMode(36)"},
+		{36, "Unknown(36)"},
 	} {
 		tcm := test.in
 		assert.Equal(t, test.want, tcm.String(), test.in)