core: support percentage specifications on TasksMax=

This adds support for a TasksMax=40% syntax for specifying values relative to
the system's configured maximum number of processes. This is useful in order to
neatly subdivide the available room for tasks within containers.

src/basic/util.h

@@ -186,6 +186,9 @@ int namespace_enter(int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int
 uint64_t physical_memory(void);
 uint64_t physical_memory_scale(uint64_t v, uint64_t max);
 
+uint64_t system_tasks_max(void);
+uint64_t system_tasks_max_scale(uint64_t v, uint64_t max);
+
 int update_reboot_parameter_and_warn(const char *param);
 
 int version(void);