dpkg-gencontrol: Emit a warning when using the deprecated -is/-ip options

Let users know these are without effect.

debian/changelog

@@ -88,6 +88,8 @@ dpkg (1.17.11) UNRELEASED; urgency=low
   * Delete the current compressor SIGPIPE disposition in Dpkg::Source::Archive,
     which fixes ignoring SIGPIPE from the calling process. Closes: #756526
   * Ignore DEB_CHECK_COMMAND in dpkg-buildpackage if the command is not found.
+  * Emit a warning when using the deprecated -is/-ip/-isp/-ips options in
+    dpkg-gencontrol.
 
   [ Updated programs translations ]
   * Danish (Joe Dalton). Closes: #754127

scripts/dpkg-gencontrol.pl

@@ -115,8 +115,8 @@ while (@ARGV) {
         $stdout= 1;
     } elsif (m/^-O(.+)$/) {
         $outputfile = $1;
-    } elsif (m/^-i[sp][sp]?$/) {
-	# ignored for backwards compatibility
+    } elsif (m/^-i([sp][sp]?)$/) {
+        warning(_g('-i%s is deprecated; it is without effect'), $1);
     } elsif (m/^-F([0-9a-z]+)$/) {
         $changelogformat=$1;
     } elsif (m/^-D([^\=:]+)[=:]/p) {