[Logic769]

When stdin is a seekable regular file that has already been partly read and is
larger than st_blksize, head -n -N and head -c -N treated stdin as
starting at byte 0. head -n -N re-printed the consumed leading bytes and
head -c -N printed part of the tail it should have dropped, and stdin was left
at the wrong offset.

head_backwards_on_seekable_file now computes its counts from the current file
offset instead of absolute offsets, and find_nth_line_from_end searches only
the data after the current position. A partly consumed seekable stdin is
therefore handled like GNU head, while file operands and pipes are unchanged.

Tests cover both -n -N and -c -N on a large file whose first line has
already been consumed.

Closes #15756

[sylvestre]

@Logic769 as I asked you many time to stop pushing a lot of patches and you ignored me, i have no other solutions than to block you temporarily.