Fix latent side-effects of respecting field boundaries in text conversion

Fix latent side-effects of respecting field boundaries in text conversion

* src/textconv.c (textconv_query, get_extracted_text)
(get_surrounding_text): Restore selected window in addition to
excursion.
(locate_and_save_position_in_field): Restore excursion lest the
current buffer differ from the selected window's contents, to
prevent redisplay from clobbering the current buffer when called
while the current buffer is at variance with the selected
window's contents, as seen with dired-other-window.