sftp: add open functions with custom attribute support

Before this patch, libssh2 sent hardcoded `LIBSSH2_SFTP_ATTRIBUTES`
struct on handle open. This can be problematic on some special OS,
where the file size should be known on new file creation. I added
two new functions to resolve this issue.

Patch-by: @vajdaakos on github via #506

Changes compared to #506:
- drop attr size fixup in favour of #946.
- move `memcpy()` under the state where we need it.
- bump filename length type to `size_t`.
- fix filenames in documentation and other nits.

Closes #506
Closes #947

docs/libssh2_sftp_open_ex_r.3

@@ -0,0 +1,73 @@
+.TH libssh2_sftp_open_ex_r 3 "10 Apr 2023" "libssh2 1.11.0" "libssh2 manual"
+.SH NAME
+libssh2_sftp_open_ex_r - open filehandle for file on SFTP.
+.SH SYNOPSIS
+.nf
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+
+LIBSSH2_SFTP_HANDLE *
+libssh2_sftp_open_ex_r(LIBSSH2_SFTP *sftp, const char *filename,
+                       size_t filename_len,
+                       unsigned long flags,
+                       long mode,
+                       int open_type,
+                       LIBSSH2_SFTP_ATTRIBUTES *attrs);
+.fi
+.SH DESCRIPTION
+\fIsftp\fP - SFTP instance as returned by \fIlibssh2_sftp_init(3)\fP
+
+\fIfilename\fP - Remote file/directory resource to open
+
+\fIfilename_len\fP - Length of filename
+
+\fIflags\fP - Any reasonable combination of the LIBSSH2_FXF_* constants:
+.RS
+.IP LIBSSH2_FXF_READ
+Open the file for reading.
+.IP LIBSSH2_FXF_WRITE
+Open the file for writing.  If both this and LIBSSH2_FXF_READ are specified,
+the file is opened for both reading and writing.
+.IP LIBSSH2_FXF_APPEND
+Force all writes to append data at the end of the file.
+.IP LIBSSH2_FXF_CREAT,
+If this flag is specified, then a new file will be created if one does not
+already exist (if LIBSSH2_FXF_TRUNC is specified, the new file will be
+truncated to zero length if it previously exists)
+.IP LIBSSH2_FXF_TRUNC
+Forces an existing file with the same name to be truncated to zero length when
+creating a file by specifying LIBSSH2_FXF_CREAT. LIBSSH2_FXF_CREAT MUST also
+be specified if this flag is used.
+.IP LIBSSH2_FXF_EXCL
+Causes the request to fail if the named file already exists.
+LIBSSH2_FXF_CREAT MUST also be specified if this flag is used.
+
+.RE
+\fImode\fP - POSIX file permissions to assign if the file is being newly
+created. See the LIBSSH2_SFTP_S_* convenience defines in <libssh2_sftp.h>
+
+\fIopen_type\fP - Either of LIBSSH2_SFTP_OPENFILE (to open a file) or
+LIBSSH2_SFTP_OPENDIR (to open a directory).
+
+\fIattrs\fP - Pointer to LIBSSH2_SFTP_ATTRIBUTES struct. See
+libssh2_sftp_fstat_ex for detailed usage.
+
+.SH RETURN VALUE
+A pointer to the newly created LIBSSH2_SFTP_HANDLE instance or NULL on
+failure.
+.SH ERRORS
+\fILIBSSH2_ERROR_ALLOC\fP -  An internal memory allocation call failed.
+
+\fILIBSSH2_ERROR_SOCKET_SEND\fP - Unable to send data on socket.
+
+\fILIBSSH2_ERROR_SOCKET_TIMEOUT\fP -
+
+\fILIBSSH2_ERROR_SFTP_PROTOCOL\fP - An invalid SFTP protocol response was
+received on the socket, or an SFTP operation caused an errorcode to be
+returned by the server.
+
+\fILIBSSH2_ERROR_EAGAIN\fP - Marked for non-blocking I/O but the call would
+block.
+.SH SEE ALSO
+.BR libssh2_sftp_close_handle(3)
+.BR libssh2_sftp_fstat_ex(3)