* sign.c (sign_file): Do not push textmode filter onto an unopened IOBUF

(segfault).  Noted by Marcus Brinkmann.  Push and reinitialize textmode
filter for each file in a multiple file list.

* packet.h, getkey.c (fixup_uidnode), keyedit.c (show_prefs): Set and show
the keyserver no-modify flag.

* keygen.c (add_keyserver_modify): New. (keygen_upd_std_prefs): Call it
here. (keygen_set_std_prefs): Accept "ks-modify" and "no-ks-modify" as
prefs to set and unset keyserver modify flag.

g10/packet.h

@@ -179,6 +179,7 @@ typedef struct {
     u32 expiredate;       /* expires at this date or 0 if not at all */
     prefitem_t *prefs;    /* list of preferences (may be NULL)*/
     int mdc_feature;
+    int ks_modify;
     u32 created;          /* according to the self-signature */
     byte selfsigversion;
     char name[1];