Fix child reaping on fuzzer termination

This commit contains the following changes:
    - Call `waitpid()` on the child and the fork server when terminating the
      fuzzer; thus, we do not end up with zombies.
    - Rename `fsrv.kill_signal` to `fsrv.child_kill_signal`, since the
      documentation states that the signal is used to terminate the *child*.
    - Use SIGTERM instead of fsrv.(child)_kill_signal, thus the fork server
      can always reap the child.

src/afl-fuzz-init.c

@@ -2963,4 +2963,3 @@ void save_cmdline(afl_state_t *afl, u32 argc, char **argv) {
   *buf = 0;
 
 }
-