Fix a problem with restarting 'tags-search'

Fix a problem with restarting 'tags-search'

* lisp/progmodes/etags.el (tags-search, tags-query-replace): Link
to 'fileloop-continue' instead of 'tags-loop-continue', for
continuing TAGS-based search/replace commands.

* lisp/fileloop.el (fileloop-continue): Reset
'switch-to-buffer-preserve-window-point' to nil when switching to
another buffer, so as to make sure a new search always restarts
from point-min in each buffer it searches.  (Bug#48628)