Added Conflicts/Replaces to smoother upgrades from distro-provided packages.

debian/nginx.control.in

diff --git a/debian/nginx.control.in b/debian/nginx.control.in
index ae17f06..8eb7068 100644
--- a/debian/nginx.control.in
+++ b/debian/nginx.control.in
@@ -17,7 +17,8 @@
 Architecture: any
 Depends: ${misc:Depends}, ${shlibs:Depends}, lsb-base (>= 3.0-6), adduser
 Provides: httpd
-Conflicts: nginx-common
+Conflicts: nginx-common, nginx-core
+Replaces: nginx-common, nginx-core
 Description: high performance web server
  nginx [engine x] is an HTTP and reverse proxy server, as well as
  a mail proxy server.