User defined kill signal value (#678)

* Adding AFL_KILL_SIGNAL environment variable

Controlling the kill signal used to end forked processes.

* Checking validity of AFL_KILL_SIGNAL env variable

This commit also sets a valid value in the environment to avoid
duplicating code in at_exit(). Changing data type of
fsrv->kill_signal to u8 to match last_kill_signal.

* Adding afl_kill_signal to AFL (environment) state

This commit simply introduces a struct member for future use. The
env variable is not used from the afl struct but from fsrv, where
its validity is checked, resulting in a FATAL in case of errors.

include/afl-fuzz.h

@@ -381,7 +381,7 @@ typedef struct afl_env_vars {
       *afl_hang_tmout, *afl_forksrv_init_tmout, *afl_skip_crashes, *afl_preload,
       *afl_max_det_extras, *afl_statsd_host, *afl_statsd_port,
       *afl_crash_exitcode, *afl_statsd_tags_flavor, *afl_testcache_size,
-      *afl_testcache_entries;
+      *afl_testcache_entries, *afl_kill_signal;
 
 } afl_env_vars_t;