Avoid `defconst` for vars which we modify

Avoid `defconst` for vars which we modify

If we `setq` or let-bind a var, then presumably it's not a const.

* lisp/bookmark.el (bookmark-bmenu-buffer):
* lisp/char-fold.el (char-fold-table):
* lisp/pcmpl-linux.el (pcmpl-linux-fs-modules-path-format)
(pcmpl-linux-mtab-file):
* lisp/emacs-lisp/bytecomp.el (byte-compile-log-buffer):
* lisp/emacs-lisp/check-declare.el (check-declare-warning-buffer):
* lisp/emacs-lisp/ert-x.el (ert-remote-temporary-file-directory):
* lisp/erc/erc.el (erc-default-port):
* lisp/net/tramp.el (tramp-unknown-id-string)
(tramp-unknown-id-integer):
* lisp/url/url-util.el (url-unreserved-chars):

(cherry picked from commit 1870e2f48a7874b9a7cd627198a6079d6a3b70c0)