Don’t trust st_size when scanning file head+tail

Don’t trust st_size when scanning file head+tail

* src/fileio.c (Finsert_file_contents): Do not look at st_size
when scanning the file’s head and tail for a coding system comment.
Instead, just use read and SEEK_END and don’t worry if the latter fails.
st_size and lseek might not work in a /proc file system.

(cherry picked from commit 0a3c8a4df3f9e15737e4e5b4aecd2e63d2abbc4f)