Sig expiration code

Offer to expire a key signature when the key the user is signing expires
Expired sigs cause an error return
If --expert is set, prompt for sig duration

g10/main.h

@@ -98,6 +98,7 @@ void keyedit_menu( const char *username, STRLIST locusr, STRLIST cmds,
 							    int sign_mode );
 
 /*-- keygen.c --*/
+u32 ask_expire_interval(int object);
 u32 ask_expiredate(void);
 void generate_keypair( const char *fname );
 int keygen_set_std_prefs (const char *string);