Full support for `/proc/sys/kernel/pid_max` changing during a process changing at runtime

[richardstartin]

Currently the profiler assumes that the value stored at /proc/sys/kernel/pid_max does not increase at runtime. The value is used for sizing various internal data structures, for example:

• the number of perf event subscriptions (how many threads can be profiled)
• the size of wallclock thread filter
• the size of the trace context lookup

In some scenarios users may increase this value during a process's lifetime which violates this assumption. Currently there is a workaround to floor the value read from /proc/sys/kernel/pid_max at 131072, but profile sample labelling would be broken (but not crash) if this number of threads is ever exceeded.