Convert some defvars to defcustoms.

Convert some defvars to defcustoms.

* lisp/speedbar.el (speedbar-ignored-modes, speedbar-file-unshown-regexp)
(speedbar-update-flag, speedbar-fetch-etags-command)
(speedbar-fetch-etags-arguments):
* lisp/term.el (term-buffer-maximum-size, term-input-chunk-size)
(term-completion-autolist, term-completion-addsuffix)
(term-completion-recexact, term-completion-fignore):
* lisp/term/sup-mouse.el (sup-mouse-fast-select-window):
* lisp/term/x-win.el (x-select-request-type):
Convert some defvars with "*" to defcustoms.