Various Debian packaging fixes

* Bump to debhelper compatibility level 9
* Update Standards-Version to 3.9.3
* Move custom install statements under dh_auto_install to an install file.
* Add a Homepage field.
* Reformat debian/copyright to match the new copyright format and to
  clarify that the license is the Expat variant.
* Add dependency on the phpapi virtual package.
* Switch to the 3.0 (native) source format.

Change-Id: I11a45de8b28a180373cde59b116258f65a457122