[3.12] gh-95782: Fix io.BufferedReader.tell() etc. being able to return offsets < 0 (GH-99709)

[miss-islington]

lseek() always returns 0 for character pseudo-devices like
/dev/urandom (for other non-regular files, e.g. /dev/stdin, it
always returns -1, to which CPython reacts by raising appropriate
exceptions). They are thus technically seekable despite not having seek
semantics.

When calling read() on e.g. an instance of io.BufferedReader that
wraps such a file, BufferedReader reads ahead, filling its buffer,
creating a discrepancy between the number of bytes read and the internal
tell() always returning 0, which previously resulted in e.g.
BufferedReader.tell() or BufferedReader.seek() being able to return
positions < 0 even though these are supposed to be always >= 0.

Invariably keep the return value non-negative by returning
max(former_return_value, 0) instead, and add some corresponding tests.
(cherry picked from commit 26800cf)

Co-authored-by: 6t8k 58048945+6t8k@users.noreply.github.com

• Issue: Negative _io.BufferedReader.tell #95782