client: Introduce new ifmatcher flag ignore_startmode

Default FALSE. Purpose to accept all kind of STARTMODE
configurations in ifstatus.
ifstatus: Accept STARTMODE={off,manual}
ifup, ifreload: rename force parameter to ignore-prio

It makes more sense and also avoid generating unwanted <config> node
in case of ifup|ifreload...
ifup: Add --ignore_startmode option to ignore STARTMODE
ifup: disable ignore_prio and ignore_startmode by default

Introduce compile time flag DEBUG_BUILD to enable them
in order to facilitate testing.