Fix octave-inf completion problems reported by Alexander Klimov.

Fix octave-inf completion problems reported by Alexander Klimov.
* lisp/progmodes/octave-inf.el (inferior-octave-mode-syntax-table):
Inherit from octave-mode-syntax-table.
(inferior-octave-mode): Set info-lookup-mode.
(inferior-octave-completion-at-point): New function.
(inferior-octave-complete): Use it and completion-in-region.
(inferior-octave-dynamic-complete-functions): Use it as well, and use
comint-filename-completion.
* lisp/progmodes/octave-mod.el (octave-mode-syntax-table): Use _ syntax for
symbol elements which shouldn't be word elements.
(octave-font-lock-keywords, octave-beginning-of-defun)
(octave-function-header-regexp): Adjust regexps accordingly.
(octave-mode-map): Also use info-lookup-symbol for C-c C-h.