core/transaction: add job mode "lenient" as an even weaker version of _FAIL

src/basic/unit-def.c

@@ -341,6 +341,7 @@ DEFINE_STRING_TABLE_LOOKUP(notify_access, NotifyAccess);
 
 static const char* const job_mode_table[_JOB_MODE_MAX] = {
         [JOB_FAIL]                 = "fail",
+        [JOB_LENIENT]              = "lenient",
         [JOB_REPLACE]              = "replace",
         [JOB_REPLACE_IRREVERSIBLY] = "replace-irreversibly",
         [JOB_ISOLATE]              = "isolate",

src/basic/unit-def.h

@@ -282,6 +282,7 @@ typedef enum NotifyAccess {
 
 typedef enum JobMode {
         JOB_FAIL,                 /* Fail if a conflicting job is already queued */
+        JOB_LENIENT,              /* Fail if any conflicting unit is active (even weaker than JOB_FAIL) */
         JOB_REPLACE,              /* Replace an existing conflicting job */
         JOB_REPLACE_IRREVERSIBLY, /* Like JOB_REPLACE + produce irreversible jobs */
         JOB_ISOLATE,              /* Start a unit, and stop all others */