version: update user-facing version indicators

- VIM_VERSION_LONG and VIM_VERSION_LONG_DATE do not affect plugins,
  they are only cosmetic.
- Vim-related version identifiers will be preserved for plugin
  compatibility (has("patchXXX") and v:version).
version: remove "Included patches" message

It is meaningless to Nvim.
version: remove non-functional Vim version identifiers
version: report commit hash as "build number" in long version string
version: stub has("nvim-1.2.3")