Improve newsticker file name handling and printing

Improve newsticker file name handling and printing

* lisp/net/newst-backend.el (newsticker--images-dir)
(newsticker--icons-dir, newsticker--cache-dir): Don't use concat
to construct file names.
(newsticker-download-images, newsticker-download-enclosures): Ditto.
(newsticker--cache-save-feed): Bind print-level etc when printing.

* lisp/net/newst-treeview.el (newsticker-treeview-save): Ditto.

This patch was mostly written by mailj2@protonmail.com, but was then
rewritten by me, and has some contributions from Xavier Capaldi (which
doesn't need copyright assignment).