xC: improve backlog helper capabilities

Snippets now receive positional parameters in the form of the buffer's
name in the locale encoding, and a filename if applicable
(we keep passing stdin along with the filename, which happens to
work out well for less(1)).

The default value of the configuration option also no longer uses
the "long prompt", which used to unhelpfully tell position in terms
of lines, but rather sets its own prompt that counts pages,
and makes sure to indicate the source buffer.

The main motivation behind this change is to make the 'v' command
work in less(1).  LESSSECURE must be omitted from the snippet
for this to work.

Bump liberty to receive a config parser that allows for less
convoluted escaping.

NEWS

@@ -1,5 +1,7 @@
 Unreleased
 
+ * xC: improved backlog helper integration capabilities
+
  * xC: made it show WALLOPS messages, as PRIVMSG for the server buffer
 
  * xD: implemented WALLOPS, choosing to make it target even non-operators