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