Eliminate make_cleanup_ui_file_delete

And now that ui_file_as_string is in, let's eliminate it.  :-)

Makes ui_file a real C++ hierarchy.  mem_fileopen is replaced with a
new string_file class that is treated as a value class created on the
stack.  This alone eliminates most make_cleanup_ui_file_delete calls,
and, simplifies code a whole lot (diffstat shows almost 1k loc
dropped.)

string_file has a string() method that gives you a direct reference to
the internal std::string.  This is what replaces old (well, new)
ui_file_as_string, which used to alway return a new copy of the same
data the stream had inside..  With direct access via a writable
reference, we can instead move the string out of the string_stream.

Note I needed a tweak on scoped_restore.  That one should probably be
split out to a separate patch.

This exposed the need to make use of gnulib namespace support.
Otherwise, making use of read/write/printf/puts/etc symbol names will
clash on systems where gnulib replaces those functions, due to
'#define foo rpl_foo'.