Attempting to put an end to the linux<->freebsd confusion.

kern_accept() used to return a newly allocated sockaddr struct instead
of modifying the given one. this is a porting issue, since we're not
copying the struct to the user when kern_accept finish.

change the calling convention of kern_accept so it'll modify the given
sockaddr struct instead of returning a newly allocated one.

the kern_xxx functions are needed by the linux compatibility layer.

previousely, the len argument weren't handled correctly, it should be returned
to the user and the sockaddr salen should be initialized with the api when
possible.