custom-file.el: put elisp comments into :comment tags

In order that the comments remain. When customization automatisms write the file, they discard elisp comments I manually inserted into my custom-file.el.
custom-file.el: made italic face explicitely non-underlined

Probably due to a bug (see http://debbugs.gnu.org/cgi/bugreport.cgi?bug=12349), italic by default automatically has underlined=on applied. So I have to turn off underline explicitely. Meaning to now break occurences where another face explicitely wants to turn underline on.