Add puts_unfiltered method to ui_file

When the pager is rewritten as a ui_file, gdb will still need a way to
bypass the filtering.  After examining a few approaches, I chose this
patch, which adds a puts_unfiltered method to ui_file.  For most
implementations of ui_file, this will just delegate to puts.  This
patch also switches printf_unfiltered to use the new method.