patch 9.1.0755: quickfix list does not handle hardlinks well

patch 9.1.0755: quickfix list does not handle hardlinks well

Problem:  quickfix list does not handle hardlinks well
Solution: store original file name with quickfix entry
          (Austin Chang)

Quickfix list shows entries with duplicate name if the file is opened
with the path of hard links.

The major cause is that qflist assumed that the filename passed into
`qf_add_entry` matches the filename opened with the buffer.

This patch handles this case by introduce a `qf_fname` into `qfline_S`
structure. It stores the filename from `qf_add_entry` for each quickfix
line.

closes: #15687

Signed-off-by: Austin Chang <austin880625@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>