This commit allows the working directory for the (old) Server
implementation to be changed without doing a `os.Chdir` first.
The feature can be enabled with `sftp.WithServerWorkingDirectory(dir)`
passed as an option to `sftp.NewServer`.
It is useful when the `sftp` is used as part of a larger service that
does more than just serve `sftp` and using `os.Chdir` is not an option.
The fallback behavior (when the option is not specified) is that the
path remains unmodified (as before).
The fakeFileInfoSys and testOsSys funcs are only used in tests, so
rename the files they are defined in to request_*_test.go to exclude
them from non-test builds.
Also rename request-example.go to request_example_test.go to exclude it
as well, since it uses said funcs in example code.
Mathias Fredriksson
Cassondra Foesch
Tobias Klauser
John Eikenberry