Fix use-after-free by tun thread after tun obj destroyed

The main thread calls pthread_cancel before freeing the tun object.
However, pthread_cancel doesn't kill the thread synchronously (man
pthread_cancel). Hence, the tun thread may still be running for a while
after the tun object is/has been(ing) freed.
Let's avoid this by making sure the thread is stopped before
freeing the object.
To accomplish it, we must wait for the thread to be cancelled. A cleanup
routie is added which will signal the "tun_released" message to the main
thread through an osmo_itq, which will then free the object (since
talloc context is managed by the main thread).

Related: SYS#5523
Change-Id: Idf005359afb41d3413b09281a9ff937d5eafcc7c
---
M daemon/daemon_vty.c
M daemon/internal.h
M daemon/main.c
M daemon/tun_device.c
4 files changed, 84 insertions(+), 15 deletions(-)