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/)

The size of long varies depending on architecture, in contrast to the
MAXLNUM constant which sets the maximum allowable number of lines to
2^32-1. This discrepancy may lead to hard to detect bugs, for example
https://github.com/neovim/neovim/issues/18454. Setting linenr_T to a
fix maximum size of 2^32-1 will prevent this type of errors in the
future.

Also change the variables `amount` and `amount_after` to be linenr_T
since they're referring to "the line number difference" between two
texts.

Verified with -Wundef

Fixes #16392

forkpty is missing on Solaris < 11 and Illumos, provide fallback implementation
for non Solaris 11 users.

Older SunOS systems come without strnlen, add STRNLEN macro in line with
the other str* compat macros.

Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>
Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>

Problem:
has('wsl') is decided at build-time.

Solution:
Check os_uname().
Fixes #12642, #16143

Problem:    Cannot parse a date/time string.
Solution:   Add strptime(). (Stephen Wall, closes #)
https://github.com/vim/vim/commit/10455d43fef041309ce0613fa792c635dd71e3a8

N/A patches for version.c:

vim-patch:8.1.2344: Cygwin: warning for using strptime()

Problem:    Cygwin: warning for using strptime().
Solution:   Move defining _XOPEN_SOURCE and __USE_XOPEN to vim.h. (Ken Takata,
            closes vim/vim#5265)  Use 700 for _XOPEN_SOURCE for mkdtemp().
https://github.com/vim/vim/commit/6a228c6463935a73c8f21142cb7368545cfee317