Prefer explicit function calls to constructor optional arguments
	to set non-mandatory attributes (e.g. trace flags).

	* src/vm/cpu.hh, src/vm/cpu.cc, src/vm/virtual_machine.hh,
	* dev/inst-builder-gen.py (Cpu, VirtualMachine, ProgramBuilder): 
	Remove uncomprehensible boolean arguments from constructor, add
	corresponding switches.
	* src/vm-tasks.cc, src/shell/shell.cc, src/parse/libparse.cc:
	Update accordingly.