copy: rework how we determine the number of bytes to copy in copy_bytes_full()

copy: rework how we determine the number of bytes to copy in copy_bytes_full()

Let's freshly calculate "m" on each iteration and always start with the maximum
size we can. If sendfile() is used we must adhere to its limit of
SSIZE_MAX minus the current offset. Otherwise we can copy more, i.e.
SSIZE_MAX without any restrictions.

Also, if we get too close to having copied SSIZE_MAX, let's turn off
sendfile() for the rest.