logind: update empty and "infinity" handling for [User]TasksMax (#3835)

The parsing functions for [User]TasksMax were inconsistent.  Empty string and
"infinity" were interpreted as no limit for TasksMax but not accepted for
UserTasksMax.  Update them so that they're consistent with other knobs.

* Empty string indicates the default value.
* "infinity" indicates no limit.

While at it, replace opencoded (uint64_t) -1 with CGROUP_LIMIT_MAX in TasksMax
handling.

v2: Update empty string to indicate the default value as suggested by Zbigniew
    Jędrzejewski-Szmek.

v3: Fixed empty UserTasksMax handling.

src/basic/cgroup-util.h

@@ -112,6 +112,10 @@ static inline bool CGROUP_BLKIO_WEIGHT_IS_OK(uint64_t x) {
             (x >= CGROUP_BLKIO_WEIGHT_MIN && x <= CGROUP_BLKIO_WEIGHT_MAX);
 }
 
+/* Default resource limits */
+#define DEFAULT_TASKS_MAX_PERCENTAGE            15U /* 15% of PIDs, 4915 on default settings */
+#define DEFAULT_USER_TASKS_MAX_PERCENTAGE       33U /* 33% of PIDs, 10813 on default settings */
+
 /*
  * General rules:
  *