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.
Merge pull request #224 from pwieczorkiewicz/ifstatus-manual

ifup, ifstatus: cond. accept STARTMODE off and manual
utils: Add ni_string_clear()
wireless: Remove secrets as soon as they are used
util: Add ni_try_mlock()

ni_try_mlock() checks for errno. It accepts EPERM error to continue,
when user is not allowed to lock memory.
wireless: mlock the network structure
wireless: catch failing network object allocation
Merge pull request #225 from pwieczorkiewicz/wireless-passwd

wireless: clean and protect passwd