Problem:
"set filetype=man" assumes the user wants :Man features, this does extra
stuff like renaming the buffer as "man://".

Solution:
- old entrypoint was ":set filetype=man", but this is too presumptuous #15487
- make the entrypoints more explicit:
  1. when the ":Man" command is run
  2. when a "man://" buffer is opened
- remove the tricky b:man_sect checks in ftplugin/man.vim and syntax/man.vim
- MANPAGER is supported via ":Man!", as documented.

fixes #15487

Update runtime files.
https://github.com/vim/vim/commit/56994d215815139207f3c5ce02a1720e44e93c09

Update runtime files
https://github.com/vim/vim/commit/130cbfc31235c6cb52ffe718ea0a5bb50fbbc9fd

Update runtime files.
https://github.com/vim/vim/commit/cb80aa2d53e56d3aba3b3c439fb467f29a750c5e

Omit runtime/doc/tabpage.txt.
Patch v8.2.1401 is not ported yet.

Port optwin.vim changes without gettext().
Patch v8.2.1544 is not ported yet.

Update runtime files.
https://github.com/vim/vim/commit/1d59aa1fdfb191d9872ff87eb94652acd374b293

Update runtime files
https://github.com/vim/vim/commit/73fef33014dbf21fc59e7e47fb091117868d82fb

Omit usr_46.txt.

Update runtime files
https://github.com/vim/vim/commit/47e13953ffdbb9f163b901196dec8c2100b72edd

Ignore *.rej files, generated by vim-patch.sh.
Source of mistakes for 1st-time contributors.

Update runtime files
https://github.com/vim/vim/commit/2cfb4a2a7248eeb40112bb482ab5b15f01b20433

Update runtime files.
https://github.com/vim/vim/commit/2963456ff2b740244b3a064785fe681b1998d75e

Update runtime files.
https://github.com/vim/vim/commit/09c6f265b21065ffa9437837b1d0955137175e45

Omit getmousepos().
Patch v8.1.2304 is not ported yet.