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.