quote unknown header keywords in diagnostics

* src/xheader.c (decx): Quote unknown header in warning, as it may
contain control characters.  Problem reported by Wicher Minnaard.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2023-08-21
+GNU tar NEWS - User visible changes. 2023-09-10
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 version TBD
@@ -31,6 +31,8 @@ used, command output will be parsed using strptime(3).
 
 ** tar no longer uses alloca, fixing an unlikely stack overflow.
 
+** When diagnosing invalid extended headers tar now quotes control characters.
+
 
 version 1.35 - Sergey Poznyakoff, 2023-07-18
 

THANKS

@@ -535,6 +535,7 @@ Warner Losh		imp@boulder.parcplace.com
 Warren Dodge		warrend@sptekwv3.wv.tek.com
 Wayne Christopher	wayne@icemcfd.com
 Werner Almesberger	werner.almesberger@lrc.di.epfl.ch
+Wichar Minnaard		wicher@gavagai.nl
 William Bader		william@nscs.fast.net
 William J. Eaton	wje@hoffman.rstnu.bcm.tmc.edu
 William Kucharski	kucharsk@netcom.com

src/xheader.c

@@ -692,8 +692,8 @@ decx (void *data, char const *keyword, char const *value, size_t size)
     t->decoder (st, keyword, value, size);
   else
     WARNOPT (WARN_UNKNOWN_KEYWORD,
-	     (0, 0, _("Ignoring unknown extended header keyword '%s'"),
-	      keyword));
+	     (0, 0, _("Ignoring unknown extended header keyword %s"),
+	      quotearg_style (shell_escape_always_quoting_style, keyword)));
 }
 
 void