fchdir: improve use of replacement functions * m4/fchdir.m4 (gl_FUNC_FCHDIR): Set appropriate witnesses. * m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Add REPLACE_FSTAT. * m4/dirent_h.m4 (gl_DIRENT_H_DEFAULTS): Add REPLACE_OPENDIR, REPLACE_CLOSEDIR. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add REPLACE_DUP. * modules/sys_stat (Makefile.am): Substitute correct witness. * modules/dirent (Makefile.am): Likewise. * modules/unistd (Makefile.am): Likewise. * lib/dirent.in.h (opendir, closedir): Use better witnesses. * lib/unistd.in.h (dup): Likewise. * lib/sys_stat.in.h (fstat): Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>