manager: Do not handle SIGKILL since we can not

This is a minor fix because it's not a major issue, this fix just avoid
to get EINVAL error from sigaction(2).

There are two signals can not handled at user space, SIGKILL and
SIGSTOP even we're PID 1, trying to handle these two signals will get
EINVAL error.

There are two kinds of systemd instance, running as system manager or
user session manager, apparently, the latter is a general user space
process which can not handle SIGKILL. The special pid 1 also can not
do that refer to kernel/signal.c:do_sigaction().

However, pid 1 is unkillable because the kernel did attach
SIGNAL_UNKILLABLE to it at system boot up, refer to
init/main.c:start_kernel()
    --> rest_init()
        --> kernel_thread()
            --> kernel_init()
                --> init_post()
                    current->signal->flags |= SIGNAL_UNKILLABLE

src/core/main.c

@@ -1405,7 +1405,6 @@ int main(int argc, char *argv[]) {
         /* Reset all signal handlers. */
         assert_se(reset_all_signal_handlers() == 0);
 
-        /* If we are init, we can block sigkill. Yay. */
         ignore_signals(SIGNALS_IGNORE, -1);
 
         if (parse_config_file() < 0)

src/shared/def.h

@@ -32,4 +32,4 @@
 #define SYSTEMD_CGROUP_CONTROLLER "name=systemd"
 
 #define SIGNALS_CRASH_HANDLER SIGSEGV,SIGILL,SIGFPE,SIGBUS,SIGQUIT,SIGABRT
-#define SIGNALS_IGNORE SIGKILL,SIGPIPE
+#define SIGNALS_IGNORE SIGPIPE