getloadavg: set errno

* lib/getloadavg.c: Set errno when returning -1.  If no other
error number looks appropriate, set it to ENOSYS if the getloadavg
looks like it can't possibly ever work, ENOTSUP otherwise.
Suggested by Bruno Haible in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-02/msg00187.html>.

getloadavg: trim unused parts and speed up 'configure'
* NEWS: Document this.
* lib/getloadavg.c: Ignore HAVE_GETLOADAVG; this file is now
always compiled if getloadavg is absent.
Move test code to ...
* tests/test-getloadavg.c: New file, containing previous
contents of test from lib/getloadavg.c.  It also contains
suggestions by Bruno Haible in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-02/msg00186.html>.
* modules/getloadavg-tests: New file.
* m4/getloadavg.m4 (gl_GETLOADAVG): Do not check for getloadavg twice.
Do tests in the same order as they're needed for getloadavg.c.
Omit setgid-related tests that generate symbols KMEM_GROUP,
NEET_SETGID, GETLOADAVG_PRIVILEGED; nobody seems to use those any more.
Do only the tests that are needed to see whether the system has
getloadavg, moving the other tests into ...
(gl_PREREQ_GETLOADAVG): ... here.  Do not define obsolete symbol
NLIST_NAME_UNION; nobody should be using it.  Do not define
symbols C_GETLOADAVG and HAVE_GETLOADAVG; they're no longer
relevant, as the user of this module shouldn't care how getloadavg
is implemented.