csu: Ensure that errno is zero at program startup time

[markj]

Summary

A dummy application which prints errno as its first action reveals that
errno is non-zero in at least two cases: the application is linked with
libthr.so, because the thr_wake(-1) call in _thr_rtld_init() sets errno
= ESRCH, and second, the application is statically linked with libthr.a,
since libc's uexterr_ctr() runs after the exterrctl(EXTERRCTL_ENABLE)
call in init_main_thread(), so the second exterrctl() call returns EBUSY
(since it does not specify EXTERRCTLF_FORCE.

Just zero errno immediately before program startup.  Or, maybe we should
be more careful and clear errno in the precise spots where errors can
occur?

Reported by:	Greg Becker

[kib]

Also see D50997.

Note that any constructor could leave errno non-zero, libc and libthr are not more guilty than arbitrary user code there.

From this PoV, the proper test for the issue should include some code like this:

static void f(void) __attribute__((constructor));
static void
f(void)
{
   errno = 11;
}

[markj]

In D50998#1163935, @kib wrote:

Also see D50997.

You posted first and your version has a nice comment, so please go ahead and I'll commit the updated tests separately.

[markj]

Set errno from a constructor.

[kevans]

Thanks!