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")`.

This is essentially a convenience wrapper around the `pending()`
function, similar to `skip_fragile()` but more general-purpose.

Also remove `pending_win32` function as it can be replaced by
`skip(iswin())`.

Problem:    The do_set() function is much too long.
Solution:   Move setting of a string option to a separate function.
https://github.com/vim/vim/commit/4740394f230dda09d6e9337465305741d8ee4fa3

Cherry-pick some tests from Vim patch 8.2.0540.

Problem:
Dirs "config", "packaging", and "third-party" are all closely related
but this is not obvious from the layout. This adds friction for new
contributors.

Solution:
- rename config/ to cmake.config/
- rename test/config/ to test/cmakeconfig/ because it is used in Lua
  tests: require('test.cmakeconfig.paths').
- rename packaging/ to cmake.packaging/
- rename third-party/ to cmake.deps/ (parallel with .deps/)

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

As Strings and Blobs are encoded as msgpack BINs, the current ShaDa
implementation will restore global Blob variables as Strings (or msgpack
special dicts if they contain NULs).

Encode an additional element with Blob globals to differentiate them
from Strings so that we can restore them with the correct type.

Adjust variables_spec.lua's autotest() to also check for proper type.

If set the number of history saves is 0, assertions fail when inserting an entry on exit.
Dont insert an entry when the number of saves is 0 fixes the issue.

fixes #11497

Problem:    Writing viminfo fails with a circular reference.
Solution:   Use copyID to detect the cycle. (closes vim/vim#6217)
https://github.com/vim/vim/commit/5b157fe2edfdce5f77080aeac2b4a03f39eb1c1a