Add prepare() to PKGBUILD.

* Move patching logic from build() to prepare() as per PKGBUILD(5).
build(): defensively specify $srcdir while changing directories.

* Defensively specify $srcdir while changing directories instead of
  assuming build() starts with the current directory of $srcdir.
Update to trafficserver release 5.0.1

* Update to trafficserver release 5.0.1