PR16716 partial fix: Better types in 'syscall.{send,sendto}'.

PR16716 partial fix: Better types in 'syscall.{send,sendto}'.

* tapset/linux/syscalls2.stp: Fixed types and nesting in
  'syscall.{send,sendto}'. Fixed a few more types in
  'syscall.{recv,recvfrom}'.
* tapset/linux/nd_syscalls2.stp: Ditto.
* runtime/linux/compat_unistd.h: Added __NR_sendto.
* tapset/linux/aux_syscalls.stp: Be sure we have the SYS_* defines by
  including '<linux/net.h>'.
* testsuite/systemtap.syscall/send.c: New testcase.
* testsuite/systemtap.syscall/sendto.c: New testcase.
* testsuite/systemtap.syscall/recv.c: Added more testing of the 'flags'
  parameter.
* testsuite/systemtap.syscall/recvfrom.c: Ditto.
* testsuite/systemtap.syscall/recvmmsg.c: Ditto.
* testsuite/systemtap.syscall/recvmsg.c: Ditto.