Improve the check for departures from C89, and fix the departures

Improve the check for departures from C89, and fix the departures
I found.
* Makefile.maint (my-distcheck): Also check for C89 compatibility
as best we can with GCC.
* src/stat.c (PRINTF_OPTION): Omit comma before } in enum
declaration; C89 doesn't allow this.
* src/dcgen: Don't generate string literals longer than
what C89 requires support for.
* src/cut.c (usage): Don't use string literals longer than
what C89 requires support for.
* src/date.c (usage): Likewise.
* src/dd.c (usage): Likewise.
* src/du.c (usage): Likewise.
* src/ls.c (usage): Likewise.
* src/od.c (usage): Likewise.
* src/readlink.c (usage): Likewise.
* src/seq.c (usage): Likewise.
* src/shred.c (usage): Likewise.