process_backslash_command would drop the last character of the input
command on the assumption that it was a newline.  Given a non newline
terminated input file, this could result in dropping the last character
of the command.  Fix that by doing an actual test that we're removing
a newline.

While at it, allow for Windows newlines (\r\n), and suppress multiple
newlines if any.  I do not think either of those cases really occur,
since (a) we read script files in text mode and (b) the lexer stops
when it hits a newline.  But it's cheap enough and it provides a
stronger guarantee about what the result string looks like.

This is just cosmetic, I think, since the possibly-overly-chomped
line was only used for display not for further processing.  So
it doesn't seem necessary to back-patch.

Fabien Coelho, reviewed by Nikolay Shaplov, whacked around a bit by me

Discussion: https://postgr.es/m/alpine.DEB.2.20.1704171422500.4025@lancre