bin/un-backslashify-selection: Fix for newer `pterm'.

bin/un-backslashify-selection: Fix for newer `pterm'.

It seems that `pterm' is trying to be more intelligent about newline
handling.  The result is that when you select a number of lines in an
Emacs ERC buffer, sometimes the final `\' of a line isn't actually
followed by a newline.

Tweak the seddery to cope with this.  In particular, zap spaces
following a `\' along with the `\' itself.