cp: Expect EINTR while copying

Both copy_file_range() and copy_fallback() can be interrupted before
they have read anything at all, in which case they return -1 and set
errno to EINTR. If that happens, we should retry, not fail.

PR: 293028
MFC after: 1 week
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D55167