behaviour of FXP_REALPATH. (Specifically, BSD and GNU realpath(3)
disagree over whether to return success when computing the realpath
for a putative new file to be created in a valid directory. There's
no way we can tell from (say) the OpenSSH version string because
OpenSSH might have been compiled to use the local realpath _or_ its
own nonbroken one.)
[originally from svn r953]
