set to -1 all unused sockets, so FS_SET won't crash if, for example,
we request UDP on a port with no IPV4 address, after the message:

   unix_open_ch: ioctl(SIOCGIFADDR): Cannot assign requested address

While at it, I factorized some common code.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>