make mupa file loading eol-style insensitive

ticket:260
I refactored the load_mupa_file function into one that takes a file,
opens it, and then passes it to a different function that takes a stream.
AKA split a function in two, so I could more easily unit test the
mupa loading code.

Once it was unit testable, I moved multiplatform_getline out of sequence,
into its own new cpp file and changed the mupa loading code to use it instead.