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.
Tejun Heo
f50582649f