core: change TasksMax= default for system services to 15%

core: change TasksMax= default for system services to 15%

As it turns out 512 is max number of tasks per service is hit by too many
applications, hence let's bump it a bit, and make it relative to the system's
maximum number of PIDs. With this change the new default is 15%. At the
kernel's default pids_max value of 32768 this translates to 4915. At machined's
default TasksMax= setting of 16384 this translates to 2457.

Why 15%? Because it sounds like a round number and is close enough to 4096
which I was going for, i.e. an eight-fold increase over the old 512

Summary:

            | on the host | in a container
old default |         512 |           512
new default |        4915 |          2457