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

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.