Replace @rmtry shell implementation with a lua implementation

This way @rmtry is now rootdir friendly
It also allows cross installation (arm rootdir on amd64 host for example)

Reviewed by:	pormtgr (mat)
Differential Revision:	https://reviews.freebsd.org/D23618

Keywords/rmtry.ucl

@@ -2,10 +2,8 @@
 #
 # MAINTAINER:	portmgr@FreeBSD.org
 actions: []
-post-deinstall: <<EOD
-	case "%@" in
-	/*) f="%@" ;;
-	 *) f="%D/%@" ;;
-	esac
-	/bin/rm -f ${PKG_ROOTDIR}/$f 2>/dev/null || /usr/bin/true
+post-deinstall-lua: <<EOD
+	file = pkg.prefixed_path("%@")
+	-- ignore the return value and the error
+	ret, err = os.remove(file)
 EOD