fchdir: port 'open' and 'close' redefinitions to AIX 7.1
* lib/chown.c, lib/clean-temp.c, lib/copy-file.c, lib/execute.c:
* lib/fsusage.c, lib/gc-gnulib.c, lib/javacomp.c, lib/mountlist.c:
* lib/openat-proc.c, lib/pagealign_alloc.c, lib/progreloc.c:
* lib/spawn-pipe.c:
Do not #undef 'open' and 'close'. AIX 7 does '#define open open64'
and then 'int open64(const char *, int, ...);', which means the
declaration for 'open' gets lost if we later '#undef open'.
Discovered while building grep pretest 2.18.151-1c770 on AIX 7.1,
where the compilation reported the non-fatal error "In function
'openat_proc_name' ... warning: implicit declaration of function
'open'". In this case the error is relatively harmless, but in
other cases it might not be so minor.