build: support --enable-single-binary=hardlinks

build: support --enable-single-binary=hardlinks

* Makefile.am: Use ln rather than $(LN_S) for hardlinks.
* configure.ac: Accept --enable-single-binary=hardlinks.
* man/local.mk: In hardlink mode, explicitly add the
hardlink creation rule to mandeps.  Given the automake
generated dependency chain, this ensures that the hardlinks
are created _after_ the multicall binary, with `make all`
or `make check` etc.
* src/local.mk: Define the new src/coreutils_hardlinks rule,
and only depend on src/coreutils_{symlinks,shebangs} if
in those modes, so that hardlinks are created _after_
the multicall binary, and other link types before.
* NEWS: Mention the new feature.
Addresses https://github.com/coreutils/coreutils/issues/129