Unify complete-symbol with completion-at-point.

Unify complete-symbol with completion-at-point.

* bindings.el (complete-symbol): Move into minibuffer.el.

* minibuffer.el (complete-tag): Move from etags.el.  If tags
completion cannot be performed, return nil instead of signalling
an error.
(completion-at-point): Make it an alias for complete-symbol.
(complete-symbol): Move from bindings.el, and replace with the
body of completion-at-point.

* progmodes/etags.el (complete-tag): Move to minibuffer.el.

* cedet/semantic.el (semantic-mode): When enabled, add
semantic-ia-complete-symbol to completion-at-point-functions.

* cedet/semantic/ia.el (semantic-ia-complete-symbol): Return nil
if Semantic is not active.