(read_minibuf): Reset echo message strings to nil.

(read_minibuf): Reset echo message strings to nil.
(Fminibuffer_completion_help): Ditto.
(Fminibuffer_complete_word): Fix computation of
i_byte when prompts are inserted into minibuffers.
(read_minibuf): Use minibuf_prompt instead of prompt.
(read_minibuf): Add front-sticky text property for prompt.
(read_minibuf_unwind): Return Lisp_Object.
(read_minibuf): Put access to minibuffer_prompt_length
in #if PROMPT_IN_BUFFER.
(minibuffer-prompt-in-buffer): Bound if
PROMPT_IN_BUFFER.
(read_minibuf): Save the value of
minibuffer_prompt_length in minibuf_save_list.
(read_minibuf_unwind): Restore minibuffer_prompt_length.
(Fminibuffer_prompt_width): Return value from
current_buffer if PROMPT_IN_BUFFER.
(read_minibuf): If PROMPT_IN_BUFFER, insert prompt into
mini-buffer and make it read-only.
(Fminibuffer_complete_and_exit): If PROMPT_IN_BUFFER, use
minibuffer_prompt_length instead of BEGV.
(Fminibuffer_complete_word): Ditto.
(Fminibuffer_complete_word): Ditto.
(read_minibuf): Conditionally insert prompt into
buffer instead of handling prompts specially.  Change other
functions accordingly.  All such code is in #if PROMPT_IN_BUFFER.
(read_minibuf): Set MUST_BE_UPDATED_P instead of
building frame matrix.
(read_minibuf): FRAME_LEFT_SCROLL_BAR_WIDTH removed.
(read_minibuf): Build frame matrix.