head: fix processing of non-seekable input as seekable

head: fix processing of non-seekable input as seekable

* src/head.c (elide_tail_bytes_file): Ensure we don't use
st_size unless we've previously used seek() to determine
the CURRENT_POS in the seekable file.
This was seen to cause issue on FreeBSD 11 when the pipe
buffer was filled with `yes | head --lines=-0`, in which
case st_size was 64KiB while ST_BLKSIZE() was 4KiB.
Reported by Assaf Gordon.