core: make TasksMax a partially dynamic property

TasksMax= and DefaultTasksMax= can be specified as percentages. We don't
actually document of what the percentage is relative to, but the implementation
uses the smallest of /proc/sys/kernel/pid_max, /proc/sys/kernel/threads-max,
and /sys/fs/cgroup/pids.max (when present). When the value is a percentage,
we immediately convert it to an absolute value. If the limit later changes
(which can happen e.g. when systemd-sysctl runs), the absolute value becomes
outdated.

So let's store either the percentage or absolute value, whatever was specified,
and only convert to an absolute value when the value is used. For example, when
starting a unit, the absolute value will be calculated when the cgroup for
the unit is created.

Fixes #13419.

src/basic/cgroup-util.h

@@ -130,7 +130,6 @@ static inline bool CGROUP_BLKIO_WEIGHT_IS_OK(uint64_t x) {
 }
 
 /* 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 */
 
 typedef enum CGroupUnified {

src/basic/limits-util.c

@@ -99,7 +99,6 @@ uint64_t physical_memory_scale(uint64_t v, uint64_t max) {
 }
 
 uint64_t system_tasks_max(void) {
-
         uint64_t a = TASKS_MAX, b = TASKS_MAX;
         _cleanup_free_ char *root = NULL;
         int r;