nut: split interface_triggers function

nut: split interface_triggers function

Using add_interface_triggers and check_interface_up is a
better separation of concerns and the code has little in
common (no duplication of code concerns).

While we are at it make both nut-monitor and nut-server failure of
add_interface_triggers non-fatal with a logged error message.

Signed-off-by: Daniel F. Dickinson <redacted>