timeout(9): remove timeout_set_kclock(), TIMEOUT_INITIALIZER_KCLOCK()

timeout(9): remove timeout_set_kclock(), TIMEOUT_INITIALIZER_KCLOCK()

We have too many timeout(9) initialization functions and macros.
Let's slim it down and combine some interfaces.

- Remove timeout_set_kclock(), TIMEOUT_INITIALIZER_KCLOCK().
- Expand timeout_set_flags(), TIMEOUT_INITIALIZER_FLAGS() to accept
  an additional "kclock" parameter.
- Reimplement timeout_set(), timeout_set_proc() with timeout_set_flags().
- Reimplement TIMEOUT_INITIALIZER() with TIMEOUT_INITIALIZER_FLAGS().
- Update the sole timeout_set_flags() user to pass a kclock parameter.
- Update the sole timeout_set_kclock() user to call timeout_set_flags().
- Update the sole TIMEOUT_INITIALIZER_FLAGS() user to provide a kclock
  parameter.

The timeout(9) code is now a bit out of sync with the manpage.  This
will be corrected in a subsequent commit.

ok kn@