Pass command line arguments to external commands.

Any option taking a command name as its argument accepts
additional arguments as well.

* lib/wordsplit.c: New file.
* lib/wordsplit.h: New file.
* lib/Makefile.am: Add new files.
* src/system.c (xexec): New function.
(run_decompress_program): Use wordsplit.
(sys_child_open_for_compress,sys_exec_command)
(sys_exec_info_script)
(sys_exec_checkpoint_script): Use xexec to invoke external
command.

* NEWS: Update.
* doc/tar.texi: Update.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2013-01-26
+GNU tar NEWS - User visible changes. 2013-02-10
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 
@@ -50,6 +50,24 @@ Additionally, the options --xattrs-include and --xattrs-exclude allow
 you to selectively control for which files to store (or extract) the
 extended attributes.
 
+* Passing command line arguments to external commands.
+
+Any option taking a command name as its argument now accepts a full
+command line as well.  Thus, it is now possible to pass additional
+arguments to invoked programs.  The affected options are:
+
+  --checkpoint-action=exec
+  -I, --use-compress-program
+  -F, --info-script
+  --to-command
+
+Furthermore, if any additional information is supplied to such a
+command via environment variables, these variables can now be used in
+the command line itself.  Care should be taken to escape them, to
+prevent from being expanded too early, for example:
+
+  tar -x -f a.tar --info-script='changevol $TAR_ARCHIVE $TAR_VOLUME'
+
 * New configure option --enable-gcc-warnings, intended for debugging.