Abort if an input file has a newline in its name

[chepner]

If an input file contains a newline, there's no way to distinguish between
that newline and the newline the terminates the file name in the editor buffer.

While workarounds exist (for example, separate the file names with null bytes),
I'm not sure how that would affect the usability of the script without significant
editor-specific configuration.

I would recommend simply aborting if any source name contains a newline, letting
the caller deal with the issue manually.