insert-file-contents file end EOF fixes

insert-file-contents file end EOF fixes

* src/fileio.c (Finsert_file_contents):
When counting bytes at file end, don’t trust lseek to seek
to the end of the file.  Instead, read a bit after lseeking,
to make sure we get to the end, give up if we don’t get to EOF.
This works around problems on /proc filesystems
where lseek pretends the file is empty.
Update file_size_hint when reaching EOF.

(cherry picked from commit aee9b598312e282700c4eedf02b174b908479ce7)