tools/rv: Stop gracefully also on SIGTERM

Currently the userspace RV tool starts a monitor and waits for the user
to press Ctrl-C (SIGINT) to terminate and stop the monitor.
This doesn't account for a scenario where a user starts RV in background
and simply kills it (SIGTERM unless the user specifies differently).
E.g.:
 # rv mon wip &
 # kill %

Would terminate RV without stopping the monitor and next RV executions
won't start correctly.

Register the signal handler used for SIGINT also to SIGTERM.

Cc: Nam Cao <namcao@linutronix.de>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Juri Lelli <jlelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Link: https://lore.kernel.org/20250723161240.194860-3-gmonaco@redhat.com


Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>