Add a new function `libssh2_sftp_posix_rename_ex()` and
`libssh2_sftp_posix_rename()`, which implement
the posix-rename@openssh.com extension.
If the server does not support this extension, the function returns
`LIBSSH2_FX_OP_UNSUPPORTED` and it's up to the user to recover, possibly
by calling `libssh2_sftp_rename()`.
Co-authored-by: Viktor Szakats (bump to size_t)
Closes#1386
