Document new <data-dir>/system-gdbinit area

gdb/doc/ChangeLog:

        * gdb.texinfo (Installed System-wide Configuration Scripts):
        Add subsection describing the scripts now available under
        the data-dir's system-gdbbinit subdirectory.
        * NEWS: Add entry announcing the availability of system-wide
        configuration scripts for ElinOS and Wind River Linux.
mi/mi-cmd-break.c: Use xsnprintf instead of sprintf (ARI fix)

gdb/ChangeLog:

        * mi/mi-cmd-break.c (mi_argv_to_format): Use xsnprintf instead
        of sprintf.