Fix PR16580 by decoding the 'protocol' argument of 'syscall.socket{pair}'.

Fix PR16580 by decoding the 'protocol' argument of 'syscall.socket{pair}'.

* tapset/linux/aux_syscalls.stp (_sock_protocol_str): New function.
* tapset/linux/syscalls2.stp: Decode the 'protocol' argument in
  syscall.socket and syscall.socketpair with _sock_protocol_str().
* tapset/linux/nd_syscalls2.stp: Ditto.
* testsuite/systemtap.syscall/socket.c: New test.
* testsuite/systemtap.syscall/socketpair.c: Updated test to expect decoded
  'protocol' arguments.
* testsuite/systemtap.syscall/net1.c: Ditto.