Once we distribute socket protocol processing requests to different

Once we distribute socket protocol processing requests to different
processors, we no longer have a process context to refer to, so
eliminate the use of curproc in soreserve() by passing the sockbuf
resource limit all the down from the system call code to sbreserve().

Eliminate the use of curproc in unp_attach() by passing down the
fields it needs from the proc structure.  Define a pru_attach_info
structure to hold the information the attach usrreq function requires.

The thread argument to in_pcballoc() is unused, so we don't need
to pass a thread structure down to in_pcballoc().