libssh/sftp: fix resume corruption by avoiding O_APPEND with rresume

libssh/sftp: fix resume corruption by avoiding O_APPEND with rresume

Opening the remote file with O_APPEND while attempting to resume causes
all writes to be forced to EOF on servers/implementations where O_APPEND
semantics override a prior seek(). As a result, sftp_seek64() is ignored
and the resumed data is appended, duplicating/corrupting the file.

Fix by:
- Using O_WRONLY (without O_APPEND) when resume_from > 0.
- Skipping the seek entirely if remote_append mode is requested.

Closes #18952