Optional signal value for kill on timeouts in TimeoutForkserverExecutor (#461)

* Optional signal value to kill forked processes on timeout

* Cargo format

* Properly initialize TimeoutForkserverExecutor

* Added with_signal constructor

* Removed duplicate code

libafl/src/executors/forkserver.rs

@@ -378,14 +378,25 @@ pub trait HasForkserver {
 pub struct TimeoutForkserverExecutor<E: Debug> {
     executor: E,
     timeout: TimeSpec,
+    signal: Signal,
 }
 
 impl<E: Debug> TimeoutForkserverExecutor<E> {
     /// Create a new [`TimeoutForkserverExecutor`]
     pub fn new(executor: E, exec_tmout: Duration) -> Result<Self, Error> {
+        let signal = Signal::SIGKILL;
+        Self::with_signal(executor, exec_tmout, signal)
+    }
+
+    /// Create a new [`TimeoutForkserverExecutor`] that sends a user-defined signal to the timed-out process
+    pub fn with_signal(executor: E, exec_tmout: Duration, signal: Signal) -> Result<Self, Error> {
         let milli_sec = exec_tmout.as_millis() as i64;
         let timeout = TimeSpec::milliseconds(milli_sec);
-        Ok(Self { executor, timeout })
+        Ok(Self {
+            executor,
+            timeout,
+            signal,
+        })
     }
 }
 
@@ -466,7 +477,7 @@ where
             self.executor.forkserver_mut().set_last_run_timed_out(1);
 
             // We need to kill the child in case he has timed out, or we can't get the correct pid in the next call to self.executor.forkserver_mut().read_st()?
-            let _ = kill(self.executor.forkserver().child_pid(), Signal::SIGKILL);
+            let _ = kill(self.executor.forkserver().child_pid(), self.signal);
             let (recv_status_len, _) = self.executor.forkserver_mut().read_st()?;
             if recv_status_len != 4 {
                 return Err(Error::Forkserver(