Print a generated-file warning at the beginning of generated
	files.
	Suggested by Roland Levillain.

	* dev/nolimips.py (generated_by_hand): Add warning string.
	* src/parse/asm-parse.yy.gen.py,
	* src/parse/asm-scan.ll.gen.py,
	* src/inst/program_solver.gen.py,
	* src/inst/nodes.gen.py,
	* src/inst/program_builder.hh.gen.py,
	Print warning right after license.
	* src/inst/nodes.mk.gen.py: Remove a useless import.