Fix PR16667 by adding clone syscall/nd_syscall tapset support.

Fix PR16667 by adding clone syscall/nd_syscall tapset support.

* tapset/linux/syscalls.stp: Add syscall.clone probe alias and deprecate
  extra variables in syscall.fork probe alias. Split out vfork support
  from syscall.fork.
* tapset/linux/nd_syscalls.stp: Ditto.
* tapset/linux/syscalls2.stp: Added syscall.vfork probe alias.
* tapset/linux/nd_syscalls2.stp: Ditto.
* tapset/linux/syscalls.stpm: Added __syscall_gate2 macro.
* runtime/linux/compat_unistd.h: Add __NR_compat_clone defines.
* testsuite/buildok/syscalls-detailed.stp: Add syscall.clone testing and
  updated syscall.fork tests.
* testsuite/buildok/nd_syscalls-detailed.stp: Ditto.
* testsuite/buildok/syscalls2-detailed.stp: Added syscall.vfork testing.
* testsuite/buildok/nd_syscalls2-detailed.stp: Ditto.
* testsuite/systemtap.syscall/forkwait.c: Updated.
* testsuite/systemtap.syscall/clone.c: New test.
* testsuite/systemtap.syscall/vforkwait.c: Ditto.