sleep: add SleepMemMode= setting for configuring /sys/power/mem_sleep

The setting is used when /sys/power/state is set to 'mem'
(common for suspend) or /sys/power/disk is set to 'suspend'
(hybrid-sleep). We default to kernel choice here, i.e.
respect what's set through 'mem_sleep_default=' kernel
cmdline option.

src/sleep/sleep.c

@@ -237,6 +237,12 @@ static int execute(
         if (state_fd < 0)
                 return log_error_errno(errno, "Failed to open /sys/power/state: %m");
 
+        if (SLEEP_NEEDS_MEM_SLEEP(sleep_config, operation)) {
+                r = write_mode("/sys/power/mem_sleep", sleep_config->mem_modes);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to write mode to /sys/power/mem_sleep: %m");
+        }
+
         /* Configure hibernation settings if we are supposed to hibernate */
         if (SLEEP_OPERATION_IS_HIBERNATION(operation)) {
                 _cleanup_(hibernation_device_done) HibernationDevice hibernation_device = {};

src/sleep/sleep.conf

@@ -23,5 +23,6 @@
 #AllowHybridSleep=yes
 #SuspendState=mem standby freeze
 #HibernateMode=platform shutdown
+#SleepMemMode=
 #HibernateDelaySec=
 #SuspendEstimationSec=60min