Utilize GNU_FINDUTILS_FD_LEAK_CHECK environment variable
to enable/disable fd leak check for xargs the same way as
for find.
* find/defs.h: Remove prototype for fd_leak_check_is_enabled().
* find/util.c: Remove implementation of fd_leak_check_is_enabled().
* lib/fdleak.c: Add implementation of fd_leak_check_is_enabled().
* lib/fdleak.h: Add prototype for fd_leak_check_is_enabled().
* xargs/testsuite/config/unix.exp: Enable GNU_FINDUTILS_FD_LEAK_CHECK
for all tests.
* xargs/xargs.c: Execute complain_about_leaky_fds when
fd_leak_check_is_enabled returns true.
* NEWS (Improvements): Document support of GNU_FINDUTILS_FD_LEAK_CHECK
by xargs.
Copyright-paperwork-exempt: Yes