PR 6691 fixed by adding support for sys_accept4.

PR 6691 fixed by adding support for sys_accept4.
* tapset/aux_syscalls.stp(_sock_type_str): Rewrote in embedded-C and added
  socket flags support.
  (_sock_flags_str): New function.
* tapset/syscalls.stp: syscall.accept prefers to use sys_accept4 when it
  exists.  Added support for sys_accept4's 'flag' parameter.
* testsuite/systemtap.syscall/net1.c (main): Updated regular expression to
  handle the new 'flags' argument.