[RFC] Runtime: Support Arch Linux/Pacman file types.

The [Arch Linux packaging routine (PKGBUILD) for Vim](https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/vim) does the following patches to the runtime:

```bash
# patch filetype.vim for better handling of pacman related files
sed -i "s/rpmsave/pacsave/;s/rpmnew/pacnew/;s/,\*\.ebuild/\0,PKGBUILD*,*.install/" \
    "${pkgdir}"/usr/share/vim/${_versiondir}/filetype.vim
```

I changed `filetype.vim` to include most of these changes out-of-the-box; I did not add the rule for `*.install`, because that is a too generic extension (Drupal CMS uses `.install` files as well, for example).

The Arch Linux PKGBUILD also includes the following lines, but unless I'm completely mistaken, they don't do anything:

```bash
sed -i "/find the end/,+3{s/changelog_date_entry_search/changelog_date_end_entry_search/}" \
    "${pkgdir}"/usr/share/vim/${_versiondir}/ftplugin/changelog.vim

sed -i 's|messages,/var|messages,/var/log/messages.log,/var|' \
    "${pkgdir}"/usr/share/vim/${_versiondir}/filetype.vim
```