Various Debian packaging fixes

* Rewrite debian/rules using dh; bump debhelper to 9.
* Update Standards-Version to 3.9.5.
* Move custom install statements under dh_auto_install to an install file.
* Rewrite debian/copyright.
* Rewrite short/long description.
* Add dependency on the phpapi virtual package.
* Switch to the 3.0 (native) source format.

Change-Id: Ibb749e18f04abd53e04428f637c1d358296cef0e
Add packaging for the HHVM extension

Add an hhvm-fss package and add hacks to build the source twice, once
for PHP and once for HHVM.

Change-Id: Ib3931ae8a3fb9ee364b2470be15f6d7ef4aaf5b9