gpgsm: Allow to add extensions at the --gen-key prompt.

* sm/certreqgen-ui.c (gpgsm_gencertreq_tty): Add a prompt for
extensions.
--

An example for an extension would be extKeyUsage for authentication:

   2.5.29.37 n 301406082B0601050507030206082B06010505070301

sm/certreqgen-ui.c

@@ -388,6 +388,11 @@ gpgsm_gencertreq_tty (ctrl_t ctrl, estream_t output_stream)
   tty_printf (_(" (optional; end with an empty line):\n"));
   ask_mb_lines (&mb_uri, "Name-URI: ");
 
+  /* Extensions */
+  tty_printf (_("Enter extensions"));
+  tty_printf (_(" (optional; end with an empty line):\n"));
+  ask_mb_lines (&mb_uri, "Extension: ");
+
 
   /* Want a self-signed certificate?  */
   selfsigned = tty_get_answer_is_yes

sm/certreqgen.c

@@ -49,6 +49,11 @@
      Signing-Key: 68A638998DFABAC510EA645CE34F9686B2EDF7EA
      %commit
 
+   Commnn extensions:
+     ExtKeyUsage: clientAuth (suggested) serverAuth (suggested)
+     -> 2.5.29.37 n 301406082B0601050507030206082B06010505070301
+
+
 */