cp: don't hang forever when the backup destination is a fifo

[Chaganti-Reddy]

cp --backup opens the backup path for writing to copy into it, but if something's already sitting there as a fifo (or socket) that just blocks forever waiting for a reader — GNU just replaces it. removed it first in backup_dest, same as the rest of cp already does when overwriting the destination itself.

Fixes #13162

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)