Problem:
The calculation of `len` in `make_filter_cmd` for powershell falls short
by one character for the following ex command:
  :%w !sort
This command satisfies these conditions:
  - `itmp` is not null
  - `otmp` is null
__NOTE__ that other shells circumvent this bug only because of `len`
allocation for six extra characters: a pair of curly braces and four
spaces:
  https://github.com/neovim/neovim/blob/cfdb4cbada8c65aa57e69776bcc0f7b8b298317a/src/nvim/ex_cmds.c#L1551-L1554
If allocation for these six characters are removed, then bash also faces
the same bug.

Solution:
Add allocation for 6 extra bytes. 1 would do, but let's keep powershell
in sync with other shells as much as possible.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

- `:write ++p foo/bar/baz.txt` should create parent directories `foo/bar/` if
   they do not exist
    - Note: `:foo ++…` is usually for options. No existing options have
      a single-char abbreviation (presumably by design), so it's safe to
      special-case `++p` here.
- Same for `writefile(…, 'foo/bar/baz.txt', 'p')`
- `BufWriteCmd` can see the ++p flag via `v:cmdarg`.

closes #19884

Problem:    Crash when trying to use s: variable in typed command.
Solution:   Don't use the script index when not set. (Ken Takata,
            closes vim/vim#6366)

https://github.com/vim/vim/commit/8e6cbb72324b6fb25d1a9abd6cc4d102d0e5f14e

Problem:    Regexp benchmark stest is old style.
Solution:   Make it a new style test.  Fix using a NULL list.  Add more tests.
            (Yegappan Lakshmanan, closes vim/vim#5963)

https://github.com/vim/vim/commit/ad48e6c1590842ab6d48e6caba3e9250734dae27

N/A patches:
vim-patch:9.0.0829: wrong counts in macro comment

Problem:    Build failure.
Solution:   Add missing changes.

https://github.com/vim/vim/commit/3a0f092ac0dbdd4ce71f9c4abe020e89f13df36c

Omit E1176: only applicable to Vim9 script.

Co-authored-by: Bram Moolenaar <Bram@vim.org>