Use completion-at-point in verilog-mode

Use completion-at-point in verilog-mode

There were some functions in verilog-mode that implemented in-buffer
completion, but this needlessly duplicates completion-at-point
functionality, and the popup window management had problems
(see Bug #23842).  We need to keep them for backwards compatibility with
older emacs versions, but use completion-at-point if available.

* lisp/progmodes/verilog-mode.el (verilog-toggle-completions): Mark as
obsolete if completion-cycle-threshold is available.
(verilog-mode-map, verilog-menu): Bind completion-at-point and
completion-help-at-point in preference to verilog-complete-word and
verilog-show-completions, respectively.
(verilog-mode): Add verilog-completion-at-point to
completion-at-point-functions.
(verilog-completion-at-point): New function.
(verilog-show-completions, verilog-complete-word): Use it to avoid code
duplication.