Within a bracket expression, the backslash is not special,
so it does not need to be escaped.
The double backslashes within brackets were found with:
grep -o '\[[^][]*\\\\[^][]*\]' syntax{,/extra}/*rc
Also, incorporate the square brackets into some bracket expressions
by listing the closing bracket first, saving a separate regex for
those two brackets.
In many places a carriage return is not valid whitespace and should
thus not be colored as such. In some of these places a vertical tab
or form feed is maybe valid whitespace, but it would be ugly or even
wrong to color them because they are not part of the subsequent
comment or keyword.
This fixes https://savannah.gnu.org/bugs/?60456.
