insert-file-contents file shrinkage in scanback

insert-file-contents file shrinkage in scanback

* src/fileio.c (emacs_full_read): New function.
(Fcopy_file, Finsert_file_contents): Use it.
(Finsert_file_contents): Check for partial reads when scanning
backwards through the file’s tail, as this indicates the file
shrank while we read it.  Also, use emacs_full_read in other
situations where it’s simpler and should be a bit faster.

(cherry picked from commit 55f41ca3aa8fe487d10730708a7396137a2c9d18)