add more gettimeofday variants and other fixes

* added new ktap builtin functions gettimeofday_ns(), gettimeofday_ms(),
  and gettimeofday_s().
* changed the internal helper C function gettimeofday_us to
  gettimeofday_ns because we can directly call the kernel API function
  getnstimeofday.
* fixed an integer overflow in the time values in the C function
  ktap_main.

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>