fchdir: use more consistent macro convention * lib/fcntl.in.h (_gl_register_fd): Move declaration to unistd. * lib/sys_stat.in.h (rpl_fstat): Declare via make-time REPLACE_FCHDIR, rather than relying on config.h macros. * lib/unistd.in.h (fchdir): Move all fchdir internal declarations inside a single make-time REPLACE_FCHDIR block, rather than using the config.h FCHDIR_REPLACEMENT. * m4/fchdir.m4 (gl_FUNC_FCHDIR): REPLACE_FCHDIR was already AC_SUBST'd, also AC_DEFINE it. Don't define FCHDIR_REPLACEMENT. Manage fstat replacement. * m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Pick up REPLACE_FCHDIR. * modules/sys_stat (Files): Add m4/unistd_h.m4. (Makefile.am): Substitute REPLACE_FCHDIR. * lib/close.c (rpl_close): Use REPLACE_FCHDIR, not FCHDIR_REPLACEMENT. * lib/dup-safer.c (dup_safer): Likewise. * lib/dup2.c (rpl_dup2): Likewise. * lib/dup3.c (rpl_dup3): Likewise. * lib/open.c (rpl_open): Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>