patch 7.4.2017

patch 7.4.2017
Problem:    When there are many errors adding them to the quickfix list takes
            a long time.
Solution:   Add BLN_NOOPT.  Don't call buf_valid() in buf_copy_options().
            Remember the last file name used.  When going through the buffer
            list start from the end of the list.  Only call buf_valid() when
            autocommands were executed.