job_exec: fix missing `arg` of usage
Makefile: `make clean` cleans example/ too
process_group: infrastructure

create new headers for new features such as pgroup
system/process.hpp renamed to system/sys_process.hpp
system/process.hpp becomes hub of libprocess
sys: process: add kill() and this_process::sleep() family

Add kill() member function to sys::process.
This function kills a process which the instance of sys::process has.
Add sys::this_process::sleep_for() and sleep_until()
The functionality is same as std::this_thread::sleep_for and sleep_until()
Make default ctor `default` (no explicit implementation)
Use new sys::this_process::sleep_for() in example/job_executor
README: remove wrong description