Include TCP_ACCEPT_FILTER for 3.x.
Add SetSockOptStringNil method on Socket.

Since Go will not allow nil where a string is expected, there is no way
to change the implementation of existing methods to accept nil strings.
This is necessary to properly support e.g. TCP_ACCEPT_FILTER, which can
only be cleared by setting it to nil.
Add a test for SetSockOptNil.
Merge pull request #53 from jtacoma/tcp-accept-filter

Support TCP_ACCEPT_FILTER in 3.x.