server: add function to get openocd shutdown status

In the app-trace module we are polling the target in the while loops
outside of the server.c
In that loops, we need to catch ctrl+c signal by checking shutdown_openocd
status

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: Id87c709a01470bf6d3642078b160a68ca85f4406
Reviewed-on: https://review.openocd.org/c/openocd/+/7142
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>