Turbostat: Re-order code and add comments

No algorithmic change in this commit
Turbostat: Fix minor compilation warnings
Turbostat: Refactor topology probing

Only probe the topology once and store in locally
Turbostat: last re-ordering of the module
Turbostat: this code can fail in fact...
Turbostat: fix allocate_cpu_set
Turbostat: fix topology detection
Turbostat: remove superfluous function declaration
Turbostat: complex read register is now useless
Turbostat: enhance error/warning meesages
Turbostat: whitespace cleanup
Turbostat: check return value of vsnprintf
Turbostat: add configuration options in man page
Turbostat: add <Plugin /> example
Turbostat: value_list_t.plugin_instance limit is DATA_MAX_NAME_LEN
Turbostat: init_counter cannot fail anymore

Due to the new probing system, the counter initialization cannot
fail anymore. Make init_counter and initialize_counters return void
Turbostat: Do not use error code, use error messages