* lib/die.h (die): New file/function from grep/coreutils.
Note: this file will probably be migrated to gnulib.
* lib/Makefile.am (libfind_a_SOURCES): Reference it.
* xargs/xargs.c: Include die.h.
(xargs_do_exec): Use die in place of error-nonzero, thus allowing
the compiler to know that we do not fall through into the 0 (child)
case.