core: add RestartSteps= and RestartSecMax= for exponentially increasing

interval between restarts

RestartSteps= accepts a positive integer as the number of steps
to take to increase the interval between auto-restarts from
RestartSec= to RestartSecMax=, or 0 to disable it.

Closes #6129

src/core/service.h

@@ -116,6 +116,8 @@ struct Service {
         char *pid_file;
 
         usec_t restart_usec;
+        unsigned restart_steps;
+        usec_t restart_usec_max;
         usec_t timeout_start_usec;
         usec_t timeout_stop_usec;
         usec_t timeout_abort_usec;
@@ -245,6 +247,8 @@ extern const UnitVTable service_vtable;
 int service_set_socket_fd(Service *s, int fd, struct Socket *socket, struct SocketPeer *peer, bool selinux_context_net);
 void service_close_socket_fd(Service *s);
 
+usec_t service_restart_usec(Service *s);
+
 const char* service_restart_to_string(ServiceRestart i) _const_;
 ServiceRestart service_restart_from_string(const char *s) _pure_;