patch 8.2.0413: buffer menu does not handle special buffers properly

patch 8.2.0413: buffer menu does not handle special buffers properly

Problem:    Buffer menu does not handle special buffers properly.
Solution:   Keep a dictionary with buffer names to reliably keep track of
            entries.
            Also trigger BufFilePre and BufFilePost for command-line and
            terminal buffers when the name changes.