Add fallback for copy_file_range EXDEV error

[Xist12gh]

Fixes #3654

Add fallback to other copy routines in case of error in previous attempt.
I tried to keep it generic, but for now it targets EXDEV error in copy_file_range explicitly.

For efficiency sake, I kept the tiered approach to available kernel routines which was previously selected by compile flags.
To do the fallback in runtime I had to duplicate some code. The effort to refactor this went a bit above the scope of a bugfix. At least for now it keeps the change readily accessible for review.
Added some comments for the one trying to clean this up.

Functional changes:
Previously:
Try one of copy_file_range or sendfile64 in this order selected during compile time, if it fails exit tr_sys_path_copy
Use user-space copy if neither was available during compile time

Now:
First try copy_file_range if available during compile time.
Try sendfile64 if available in case of previous EXDEV or when copy_file_range wasn't available.
Then try user-space copy if neither kernel routine was available during compile time or previous EXDEV errno was set

[ckerr]

FTBFS:

/usr/home/user/buildAgent/work/1f3bff43dd55cfe9/libtransmission/file-posix.cc:491:5: error: unknown type name 'error_t'; did you mean 'errno_t'?

[kisol-pw]

Xist12gh here:
hm, as I feared, too environment specific.
well, let's not get overambitious and go with the traditional int then.
I'll push an updated when I get off work in a few hours.

[Xist12gh]

Updated PR

• rebased to 72a1696
• switched to traditional int for errno
• fixed unneeded logging of EXDEV even though the fallback catches it, somehow slipped the original testing

Let's see if this works now.

If nobody beats me to it, I'll probably refactor the duplicated parts in a later PR once this one proves to run stable.