Writing ktrace files to NFS must no be done while holding the net

Writing ktrace files to NFS must no be done while holding the net
lock.  accept(2) panics, connect(2) dead locks.  Additionally copy
in or out must not hold the net lock as it may be a memory mapped
file on NFS.
Simplify dns_portcheck(), it does not modify namelen anymore.
In doaccept() release the socket lock before calling copyaddrout().
Rearrange the checks in sys_connect() like they are in sys_bind().
OK mpi@