patch 9.2.0475: runtime(netrw): bookmark paths not normalized

patch 9.2.0475: runtime(netrw): bookmark paths not normalized

Problem:  the bookmarks list can have duplicate entries, more often
          in win32 (due to mixed slashes and capitalization) and when
          g:netrw_keepdir=0 (which could introduce relative paths).
          Duplicate entries could be: C:\foo\BAR\baz.file
                                     c:\foo\bar\baz.file
                                     c:/foo\BAR/baz.file
                                     BAR/baz.file
Solution: Normalize the paths and make sure they are always absolute
          (J. Paulo Seibt).

closes: #20194

Signed-off-by: J. Paulo Seibt <jpseibt@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>