mgmt/mcumgr/lib: Allow signed priorities in taskstat

By default the mcumgr taskstat responses send task priorities
as unsigned integers, while Zephyr uses int8_t as priority type.
This commit adds OS_MGMT_TASKSTAT_SIGNED_PRIORITY Kconfig option
that allows to switch to use signed priorities in responses.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>