Added a ClientOption to determine whetehr to use Fstat or Stat when File.WriteTo is being called to support strange behaviour on some servers

Issue 295 corrupt resumes

fix S_IFMT value on windows, plan9 and js/wasm

Fixes issue with append uploads. Was opening the file with O_APPEND, but
it uses WriteAt() to write the data which doesn't work with a file
opened in append mode. Removing the append flag fixes the issue as the
client is sending the offsets anyways.

Also added a note to the Request server's FileWriter interface on
handling append flags.

Simulates resuming a file upload where more data is appended to the same
file (eg. network error, uploading log files, etc). Currently broken in
the server as it uses O_APPEND + WriteAt() which don't work together.

Added a ClientOption to determine whetehr to use Fstat or Stat when File.WriteTo is being called to support strange behaviour on some servers

Go defines S_IFMT on windows, plan9 and js/wasm as 0x1f000 instead of
0xf000. None of the the other S_IFxyz values (even on those mentioned)
include the "1" (in 0x1f000) which prevents them from matching the
bitmask.

This fixes that by overriding the S_IFMT value on the effected platforms
to be 0xf000, as it it on all others.

Fix panics when attempting to get file attributes from too small byte slices

make SFTP extensions configurable

Thanks @drakkan

Fix posixrename and add a test case

Revert "Bug: windows specific issue on request-server.go"

Bug: windows specific issue on request-server.go

windows filepath.Dir(), more specifically the clean function preforms
incorrectly in our situation. As the filesystem is constant we can do a
simplified dir() function to resolve the issue.

selected. I am unsure if this is the final solution as it will undo a
previous update that may resolve a differen issue.

Bug with sub folders/files of a renamed/removed path

fix nil panic error

Issue 305 client write

Fixes #305

Need to get StatusError.Code (uint32) converted to fxerr type so you can
compare it to public (exported) ErrSSHFx[..] error types. This adds a
new method, FxCode(), to the StatusError objects to handle this case.

Did this vs trying to change types of existing fields to not break
backwards compatibility.

fix: received packet too long

fix lint issues and update to be more idiomatic

Close #308

notify errors to readerAt and writerAt

Add an optional interface that readerAt and writerAt can implement
to be notified about the error causing Serve() to exit with the
request still open.

Implement the TransferError interface in request-example.
This way we can run the request server example in debug mode, for example:

cd examples/request-server
go run -tags debug main.go

simulate a connection error killing an sftp client while uploading/downloading
and see the debug log that shows that the error is correctly notifyed

Fixes #306

These lint issues remain:

- request-errors.go, aliases for new error types
- request-attrs.go, UidGid. Changing this will break compatibility

define supported extensions in one place and use for both server and …