Consider replacing SFTP.rename with SFTP.posix_rename in 3.0

[bitprophet]

As per #1012 it's easy for folks caring about the details to get confused by the current, legacy behavior of SFTP file renaming. One way to solve this is for us to declare the old behavior to be bad/incorrect, and to just nuke it in favor of the 'POSIX' style rename in a major release.

I'd still like to know more about the differences & see if there's any info on why the 'non-POSIX' behavior was initially added (my guess is that it's just a naive first implementation, but dunno) but that's mostly a sanity check.

[bitprophet]

Pro tip, maybe leave ticket triage for later in the day...clearly not awake enough not to miss stuff.

We added posix_rename in 2.2, via #65 / #471 and from those tickets it's clear that the deal is the "posix rename" is an extension to the protocol implemented by OpenSSH specifically. Following from this is the assumption that other SSH/SFTP servers may well not implement this extension.

So this feels like it should just be a documentation-only "fix" (which was going to be another ticket, but now, no point) to try and help folks like the submitter of #1012 notice quicker that the behavior they are seeking does exist.