Changes

logging_vty: Set 'gsmtap log nonblocking-io' as default

Apps using a VTY are expected to be using an event loop, and hence
should not be using blocking operations which would stall the event
loop.

If user fears losing messages (eg, when enabling a lot of DEBUG),
then the taget can still be switched to "wq" at the expense of
losing performance, or increasing the kernel UDP socket sndbuf by means
of sysctl net.core.wmem_{default,max}.

Related: OS#6213
Related: OS#6794
Change-Id: Ifca8a821e13ec1327ab2476b0db91078fcff948b