Ports: Add interactive mode to .port_include.sh

Running `./package.sh interactive` in a port directory will
spawn a new shell with the serenity build environment set up.
This makes porting software much easier as build commands can
be run interactively instead of having to modify package.sh
just to test things.

Ports/.port_include.sh

@@ -547,8 +547,12 @@ parse_arguments() {
             shift
             parse_arguments $@
             ;;
+        interactive)
+            export PS1="(serenity):\w$ "
+            bash --norc
+            ;;
         *)
-            >&2 echo "I don't understand $1! Supported arguments: fetch, patch, configure, build, install, installdepends, clean, clean_dist, clean_all, uninstall, showproperty."
+            >&2 echo "I don't understand $1! Supported arguments: fetch, patch, configure, build, install, installdepends, interactive, clean, clean_dist, clean_all, uninstall, showproperty."
             exit 1
             ;;
     esac