core: allow "off" as special watchdog time to be specified

Right now we already understand "default" as special string for enabling
the watchdog but not reconfiguring its timeout (it is internally mapped
to USEC_MAX). To be systematic this adds "off" as special string for
disabling the watchdog logic (it is internally mapped to 0, which is how
this behaviour was previously requested).

src/core/load-fragment.c

@@ -6347,6 +6347,8 @@ int config_parse_watchdog_sec(
                 void *data,
                 void *userdata) {
 
+        usec_t *usec = data;
+
         assert(filename);
         assert(lvalue);
         assert(rvalue);
@@ -6354,12 +6356,12 @@ int config_parse_watchdog_sec(
         /* This is called for {Runtime,Reboot,KExec}WatchdogSec= where "default" maps to
          * USEC_INFINITY internally. */
 
-        if (streq(rvalue, "default")) {
-                usec_t *usec = data;
-
+        if (streq(rvalue, "default"))
                 *usec = USEC_INFINITY;
-                return 0;
-        }
+        else if (streq(rvalue, "off"))
+                *usec = 0;
+        else
+                return config_parse_sec(unit, filename, line, section, section_line, lvalue, ltype, rvalue, data, userdata);
 
-        return config_parse_sec(unit, filename, line, section, section_line, lvalue, ltype, rvalue, data, userdata);
+        return 0;
 }

src/core/main.c

@@ -544,6 +544,8 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
 
                 if (streq(value, "default"))
                         arg_runtime_watchdog = USEC_INFINITY;
+                else if (streq(value, "off"))
+                        arg_runtime_watchdog = 0;
                 else {
                         r = parse_sec(value, &arg_runtime_watchdog);
                         if (r < 0) {

src/core/system.conf.in

@@ -29,9 +29,9 @@
 #CPUAffinity=
 #NUMAPolicy=default
 #NUMAMask=
-#RuntimeWatchdogSec=0
+#RuntimeWatchdogSec=off
 #RebootWatchdogSec=10min
-#KExecWatchdogSec=0
+#KExecWatchdogSec=off
 #WatchdogDevice=
 #CapabilityBoundingSet=
 #NoNewPrivileges=no