ng_socket: simplify protosw(9) methods

• so_pcb can't be non-NULL in pr_attach.
• so_pcb can't be NULL in all other methods.
• Short circuit data socket attach down to the common attach.
• Short circuit both detach methods down to common one.
• Inline control socket attach.
• Inline control pr_bind.
• Inline data pr_connect.
• Rely on pr_connect_notsupp() for control connect(2).
• Don't use cast on so_pcb, it is void *.

Should be no functional change.