get_update: + FIXME items

We should let maintainers know if they are up-to-date
Fix use of :pkgver, replace with PACKAGE_VERSION

This is bc. PACKAGE_VERSION is dertemined at the time :s-makepkg
runs, while :pkgver is not (it comes from PACKAGE_VERSION)
get_update: new implementation that fixes the FIXME items

Strategy

* Get the current version (x.y.z.t) and the first patch (x.y.z)
* Fetch `sudo` homepage, and find the latest version started by `x.`
* If the latest version equals to the current version, print the
  update status, and exit (0)
* If the latest version is not the current version, this indicates
  that we have a new version to update. We provide the link to
  this update. There are two cases
  - It is on the same stream of the current version
      the latest stable version == a patch update
  - Otherwise,
      the latest stable version == a major update
      there is possibly new patch for the current version