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

include/libssh2_sftp.h

@@ -238,6 +238,17 @@ libssh2_sftp_open_ex(LIBSSH2_SFTP *sftp,
     libssh2_sftp_open_ex((sftp), \
                          (path), (unsigned int)strlen(path), \
                          0, 0, LIBSSH2_SFTP_OPENDIR)
+LIBSSH2_API 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);
+#define libssh2_sftp_open_r(sftp, filename, flags, mode, attrs) \
+    libssh2_sftp_open_ex_r((sftp), (filename), strlen(filename), \
+                           (flags), (mode), LIBSSH2_SFTP_OPENFILE, \
+                           (attrs))
 
 LIBSSH2_API ssize_t libssh2_sftp_read(LIBSSH2_SFTP_HANDLE *handle,
                                       char *buffer, size_t buffer_maxlen);