cat: improve fallback when splice failed, dedup a code

[oech3]

• dedup a code with buf_copy
• drop BUF_SIZE
• use same buf size with pipe at write fallback when splice from broker pipe failed. (this failure would not happen generally. So using 1 MiB should not be problem)
• directly use rustix::io to make its meaning clear (raw syscall usage)
• remove assertion from release binary

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[oech3]

duplicated code?

coreutils/src/uucore/src/lib/features/buf_copy/linux.rs

Line 121 in 44aff6b

/// Move exactly `num_bytes` bytes from `read_fd` to `write_fd` using the `read`