selinux-h: always use getfilecon wrappers

selinux-h: always use getfilecon wrappers

* lib/getfilecon.c: New file.
* lib/se-selinux.in.h: Use a better inclusion guard symbol name.
[HAVE_SELINUX_SELINUX_H]: Include-next <selinux/selinux.h>.
[!HAVE_SELINUX_SELINUX_H]: Use better parameter names.
(fgetfilecon): Provide a stub.
* m4/selinux-selinux-h.m4 (gl_HEADERS_SELINUX_SELINUX_H): Don't
AC_SUBST SELINUX_SELINUX_H, since now we're generating that
file unconditionally.
When <selinux/selinux.h> is found, arrange to use wrappers.
* modules/selinux-h (Files): Add getfilecon.c.
(Makefile.am): Substitute include-next-related bits
into the now-always-generated selinux/selinux.h file.
* doc/glibc-functions/getfilecon.texi: New file.
* doc/glibc-functions/lgetfilecon.texi: New file.
* doc/glibc-functions/fgetfilecon.texi: New file.
* doc/glibc-functions/getfilecon-desc.texi: New file.
* doc/gnulib.texi (Glibc selinux/selinux.h): New section, by
which to pull in the new files.
* MODULES.html.sh (Misc): Add selinux-h.