string: Fix strerrorname_np return value [BZ #26555]

It returns the string of the error constant, not its description (as
strerrordesc_np).  To handle the Hurd error mapping, the ERR_MAP was
removed from errlist.h to errlist.c.

Also, the testcase test-strerr (added on 325081b9) was not added
on the check build neither it builds correctly.  This patch also
changed it to decouple from errlist.h, the expected return values
are added explicitly for both both strerrorname_np and strerrordesc_np
directly.

Checked on x86_64-linux-gnu and i686-linux-gnu.  I also run a make
check for i686-gnu.

(cherry picked from commit cef95fdc)