Make end-of-line recognition more robust

[Goober5000]

The newline processing in the FSO parser could be improved. As seen in parse_get_line(), FSO simply ignores any carriage returns it finds (\r, character 13) and only processes a newline when it encounters a line feed (\n, character 10). But if a carriage return is not immediately followed by a line feed, two lines can be unexpectedly concatenated. The combination of this plus a semicolon caused the mysterious behavior seen in #6389.

Better behavior would be to move to a newline on a \r or \n or \r\n and to display a warning if the newline styles are mixed within the same file.