handle - SFTP File Handle as returned by
offset - Number of bytes from the beginning of file to seek to.
Move the file handles internal pointer to an arbitrary location. libssh2
implements file pointers as a localized concept to make file access appear
more POSIX like. No packets are exchanged with the server during a seek
operation. The localized file pointer is simply used as a convenience offset
during read/write operations.
You MUST NOT seek during writing or reading a file with SFTP, as the internals
use outstanding packets and changing the "file position" during transit will
results in badness.