configmode: formular enhancements

[sargon]

In case of a form error the color of the field gets red, with the current theme. Sadly the cursor is nearly the same color, so not visible anymore. This behaviour leads to confusion. Furthermore, in the case of the hostname, the error message should include information about valid character and maximal length.

[tcatm]

Showing information about valid characters would require deep changes in luci. We're just using their validation feature for hostnames.