named is broken on linux due to user permissions

Change number 5031 (GL #525 (closed)) broke named's ability to change UID for file access on linux. On ubuntu 16.04 I have a named.conf file owned by user "each" and named cannot load it, either with sudo named or sudo named -ueach. This is almost certainly due to the removal of HAVE_LINUXTHREADS code in bin/named/unix/os.c.