Decode setsockopt() multicast arguments

[Ben Noordhuis]

The first patch makes the setsockopt() option parser aware of
IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP and its IPv6 variants.
It also fixes a minor fall-through bug in the SOL_IP switch case.

The second patch reduces the line count in net.c a little by reusing
a function that is introduced in the first patch.

  net.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++------------
  1 file changed, 96 insertions(+), 23 deletions(-)