build: fall back to do_mknod() when mknodat() is unavailable (#896)

build: fall back to do_mknod() when mknodat() is unavailable (#896)

do_mknod_at() (the symlink-race-safe variant used by a non-chrooted
daemon receiver) calls mknodat()/mkfifoat(), but the at-variant was
gated only on AT_FDCWD.  Older Darwin declares AT_FDCWD without
mknodat(), so the build failed with "mknodat undeclared".

Probe mknodat()/mkfifoat() in configure and require HAVE_MKNODAT for the
at-variant; without it do_mknod_at() falls back to do_mknod(), exactly
as it already does where AT_FDCWD is missing.  Linux keeps the mknodat
path since HAVE_MKNODAT is defined there.

Fixes: #896