gpg: Request keygrip of key to add via command interface

* g10/keygen.c (ask_algo): Request keygrip via cpr_get.
* doc/help.txt (gpg.keygen.keygrip): New help text.
--

This change makes it possible to add an existing (sub)key to
another key via the status/command interface.

GnuPG-bug-id: 5771
(cherry picked from commit 19b1a28621)

doc/help.txt

@@ -134,6 +134,15 @@ Please consult your security expert first.
 .
 
 
+
+.gpg.keygen.keygrip
+Enter the keygrip of the key to add.
+
+The keygrip is a string of 40 hex digits that identifies a key.  It
+must belong to a secret key or a secret subkey stored in your keyring.
+.
+
+
 .gpg.keygen.flags
 Toggle the capabilities of the key.