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_r.3

@@ -0,0 +1,23 @@
+.TH libssh2_sftp_open_r 3 "10 Apr 2023" "libssh2 1.11.0" "libssh2 manual"
+.SH NAME
+libssh2_sftp_open_r - convenience macro for \fIlibssh2_sftp_open_ex_r(3)\fP calls
+.SH SYNOPSIS
+.nf
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+
+LIBSSH2_SFTP_HANDLE *
+libssh2_sftp_open_r(LIBSSH2_SFTP *sftp, const char *filename,
+                    unsigned long flags,
+                    long mode,
+                    LIBSSH2_SFTP_ATTRIBUTES *attrs);
+.fi
+.SH DESCRIPTION
+This is a macro defined in a public libssh2 header file that is using the
+underlying function \fIlibssh2_sftp_open_ex_r(3)\fP.
+.SH RETURN VALUE
+See \fIlibssh2_sftp_open_ex_r(3)\fP
+.SH ERRORS
+See \fIlibssh2_sftp_open_ex_r(3)\fP
+.SH SEE ALSO
+.BR libssh2_sftp_open_ex_r(3)