avoid warnings about initialization of automatic aggregates

avoid warnings about initialization of automatic aggregates

* src/system.h (DZA_CONCAT0, DZA_CONCAT): New macros.
(DECLARE_ZEROED_AGGREGATE): New macro.
* src/ls.c (quote_name): Use it.
* src/pathchk.c (portable_chars_only): Use it.
* src/shred.c (main): Use it.
* src/stty.c (main): Use it.
* src/wc.c (SUPPORT_OLD_MBRTOWC): Use it.