Make local sockets world-readable.

We already have S_IWOTH, so it's kind of pointless to not permit read
access to the socket special. This patch changes afs.c and audiod.c
to create sockets with mode 666 which was probably intended anyway.

The patch should not cause any compatibility issues since on Linux we
check credentials with SCM_CREDENTIALS while *BSD ignores permissions
for UNIX domain sockets. According to unix(7), portable programs
should not rely on them.