core: Add SERVICE_RUNNING_REVALIDATING state

For a Type=dbus service we sometimes want to keep treating it as running
while its bus name has briefly gone away (for example, due to a broker
restart).

SERVICE_RUNNING_REVALIDATING is active and keeps its main process like
SERVICE_RUNNING, but it's deliberately not part of the runtime timer
set, so RuntimeMaxSec= is paused while we sit in it.

src/basic/unit-def.c

@@ -233,6 +233,7 @@ static const char* const service_state_table[_SERVICE_STATE_MAX] = {
         [SERVICE_START]                      = "start",
         [SERVICE_START_POST]                 = "start-post",
         [SERVICE_RUNNING]                    = "running",
+        [SERVICE_RUNNING_REVALIDATING]       = "running-revalidating",
         [SERVICE_EXITED]                     = "exited",
         [SERVICE_REFRESH_EXTENSIONS]         = "refresh-extensions",
         [SERVICE_REFRESH_CREDENTIALS]        = "refresh-credentials",

src/basic/unit-def.h

@@ -132,6 +132,7 @@ typedef enum ServiceState {
         SERVICE_START,
         SERVICE_START_POST,
         SERVICE_RUNNING,
+        SERVICE_RUNNING_REVALIDATING,   /* Temporarily waiting for the D-Bus name to reappear */
         SERVICE_EXITED,                 /* Nothing is running anymore, but RemainAfterExit is true hence this is OK */
         SERVICE_REFRESH_EXTENSIONS,     /* Refreshing extensions for a reload request */
         SERVICE_REFRESH_CREDENTIALS,    /* ditto, but for credentials */