Copy the behavior of 'undodir' and create the last specified directory
in the 'backupdir' option if it doesn't exist.

Use trailing slashes for 'backupdir' as well as 'viewdir' and 'undodir'
by default. Note that 'undodir' always behaves as though it has the
trailing slashes, regardless of whether or not they are present. They
are added to the default option value to minimize surprise.

The '.' value in 'backupdir' is kept because the default behavior for
backups is solely to have a backup if the save of the main file to disk
fails. As soon as that save is completed the backup file is removed, so
generally there is no need to put them in a central location.

Co-authored by: murphy66 <murphy66@gmail.com>

while there is some controversy, stdpath('cache') looks like  a better fit for logs than stdpath('data'): you can remove logs without preventing nvim to work which fits the XDG_CACHE_HOME definition of `user specific non-essential data files`.

- Windows environment variables are semicolon-separated, but some logic
  was assuming colon (:). This broke initialization and parsing of
  XDG_CONFIG_DIRS, XDG_DATA_DIRS, 'runtimepath', stdpath(), etc.
- test/defaults_spec: enable tests on Windows

ref #12793

Problem:    'shortmess' flag "n" not used in two places.
Solution:   Make use of the "n" flag consistent. (Nick Jensen, closes vim/vim#6245,
            closes vim/vim#6244)
https://github.com/vim/vim/commit/722e505d1a55dfde5ab62241d10da91d2e10c3c1

Problem: cannot have a local value for 'scrolloff' and 'sidescrolloff'

Author: Bram Moolenar

https://github.com/vim/vim/commit/375e3390078e740d3c83b0c118c50d9a920036c7

Similar to signcolumn, allow foldcolumn to adapt itself to the number of
folds.

Regression:
vim supports a maximum fdc of 12, this limits it to 9.