net: Add code to analyze stack usage

To make it easy to periodically verify that we have sane stack sizes
it's helpful to have some code to evaluate how much of the stacks are
used. There's already a CONFIG_INIT_STACKS option which pre-fills the
stacks with 0xaa bytes, so we make use of it and in case the option is
enabled run the extra code to evaluate how much of each stack was
used. The trigger for the evaluation is a disconnection event.

Copied from Bluetooth sub-system.

Change-Id: Ic04ff83f682dc9b57afb69e164b972ffd9b860ea
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>