buf_copy: remove unnecessary flush

[xtqqczze]

• replace direct trait use with FdReadable and FdWritable traits for improved abstraction
• avoid blanket trait implementations to ensure only valid types (files or pipes) are used
• remove unnecessary .flush() since File and PipeWriter are unbuffered

Closes #12401

[oech3]

I already opened PR for this. Please close this.

[xtqqczze]

@oech3 Sorry I missed that PR, it looks like these changes are an improvement though?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout-group (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[oech3]

This PR with PipeReader is invalid:

> strace -c -e splice cp /dev/stdin /tmp/a
a
a
a
^Cstrace: Process 1128 detached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.001080         180         6           splice

[oech3]

BUGFS is broken with io::copy #9609 (comment) . So internal kernel_copy should be bypassed by RawReader.
[Edit] ignore this comment. Here is not a place we can fix it...