New program: chcon

New program: chcon

* gl/modules/selinux-at: New module.  Check for libselinux and set
LIB_SELINUX here, unconditionally, rather than depending on
the configure-time --enable-selinux option.
* gl/modules/selinux-h: New module.
* bootstrap.conf (gnulib_modules): Add selinux-at.
* gl/lib/selinux-at.c, gl/lib/selinux-at.h: New files.
* gl/lib/se-selinux_.h: New file.
* gl/lib/se-context_.h: New file.
* gl/m4/selinux-selinux-h.m4: New file.
* gl/m4/selinux-context-h.m4: New file.
* src/Makefile.am (bin_PROGRAMS): Add chcon.
(chcon_LDADD): Define.
* README: Add chcon to the list of programs.
* src/chcon.c: Rewrite the original (Red Hat) chcon to use fts.