Fix argument handling when running external commands.

* src/system.c (xexec): Use sh -c to run the command.  This fixed
bug introduced by 7b5e80396 (tar 1.27)
* doc/tar.texi: Fix checkpoint examples: (1) $TAR_FILENAME
is not available when creating archive and (2) --checkpoint
can't be used as abbreviation of --checkpoint-action

doc/tar.texi

@@ -4482,7 +4482,7 @@ command, provided that they are properly escaped, for example:
 
 @smallexample
 @kbd{tar -c -f arc.tar \
-     --checkpoint-action='exec=/sbin/cpoint $TAR_FILENAME'}
+     --checkpoint-action='exec=/sbin/cpoint $TAR_CHECKPOINT'}
 @end smallexample
 
 @noindent
@@ -4737,7 +4737,7 @@ command line of the external command.  For example:
 
 @smallexample
 $ @kbd{tar -x -f archive.tar \
-    --checkpoint=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'}
+    --checkpoint-action=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'}
 @end smallexample
 
 @noindent

src/system.c

@@ -27,13 +27,14 @@
 static _Noreturn void
 xexec (const char *cmd)
 {
-  struct wordsplit ws;
+  char *argv[4];
 
-  ws.ws_env = (const char **) environ;
-  if (wordsplit (cmd, &ws, (WRDSF_DEFFLAGS | WRDSF_ENV) & ~WRDSF_NOVAR))
-    FATAL_ERROR ((0, 0, _("cannot split string '%s': %s"),
-		  cmd, wordsplit_strerror (&ws)));
-  execvp (ws.ws_wordv[0], ws.ws_wordv);
+  argv[0] = (char *) "/bin/sh";
+  argv[1] = (char *) "-c";
+  argv[2] = (char *) cmd;
+  argv[3] = NULL;
+
+  execv ("/bin/sh", argv);
   exec_fatal (cmd);
 }