syntaxes: replace [[:space:]] with [[:blank:]] to exclude carriage return

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.

syntax/default.nanorc

@@ -5,7 +5,7 @@ syntax default
 comment "#"
 
 # Comments.
-color cyan "^[[:space:]]*#.*"
+color cyan "^[[:blank:]]*#.*"
 
 # Spaces in front of tabs.
 color ,red " +	+"