In DUMP_FUNCTION_SPECIALIZED, use isscreensafe_ucs (for UTF-8) or
isscreensafe (for unibyte) to detect control characters, and replace
them with spaces. add_document_to_string already did the same.
In DUMP_FUNCTION_SPECIALIZED (used by elinks --dump), detect the
second cell of double-cell (aka fullwidth) characters by comparing to
UCS_NO_CHAR, like add_document_to_string does. Don't use
unicode_to_cell for this any more.
Also, ignore the colors and attributes of the second cell; don't
output any escape sequences for them.
This is especially useful for showing that neither dump_truecolor_utf8
nor dump_truecolor_unibyte modifies its static color[] variable and it
therefore does not matter whether those functions use the same array
or not.
With all the comments and macros needed for this, the source files
don't become much shorter, but anyway I hope they'll be easier to
maintain this way.
