Fix dir separator problem for Windows filenames
Add default value for filename in compile.string
Remove requirement of specific luajit version in bcsave

Not needed because the generation and printing of bytecode should work
with any LuaJIT version. Yet the generated bytecode will always be for
LuaJIT 2.0.
Modify the chunkname information when input is stdin
Fix problem in bcsave with common use cases
Remove unneeded chunkname makeup in test-bytecode