PR16716 partial fix: Better types in 'syscall.{sendmsg,sendmmsg}'.

PR16716 partial fix: Better types in 'syscall.{sendmsg,sendmmsg}'.

* tapset/linux/syscalls2.stp: Fixed types and nesting in
  'syscall.{sendmsg,sendmmsg}.
* tapset/linux/nd_syscalls2.stp: Ditto.
* tapset/linux/syscalls.stpm: Added __compat_syscall() macro.
* tapset/linux/aux_syscalls.stp: Includes compat_net.h.
* runtime/linux/compat_net.h: New file.
* runtime/linux/compat_unistd.h: Added defines.
* testsuite/systemtap.syscall/sendmmsg.c: New test.
* testsuite/systemtap.syscall/sendmsg.c: Ditto.