library: Fix off-by-one error in procps_pid_length

Section "pid_max" in Linux admin-guide/sysctl/kernel.rst says:

PID allocation wrap value. When the kernel's next PID value reaches this value, it wraps back to a minimum PID value. PIDs of value "pid_max" or larger are not allocated.

In particular, when pid_max == 10**N exactly, then the largest possible PID is 10**N - 1, i.e. the length is N.

This fixes issue #412 (closed).