tidy-up: fix more nits

- fix indentation errors.
- reformat `cmake/FindmbedTLS.cmake`
- replace a macro with a variable in `example/sftp_RW_nonblock.c`.
- delete macOS macro `_DARWIN_USE_64_BIT_INODE` from the
  OS/400 config header, `os400/libssh2_config.h`.
- fix other minor nits.

Closes #983

src/misc.c

@@ -204,10 +204,10 @@ _libssh2_send(libssh2_socket_t sock, const void *buffer, size_t length,
 #else
     if(rc < 0) {
 #ifdef EWOULDBLOCK /* For VMS and other special unixes */
-      if(errno == EWOULDBLOCK)
-        return -EAGAIN;
+        if(errno == EWOULDBLOCK)
+            return -EAGAIN;
 #endif
-      return -errno;
+        return -errno;
     }
 #endif
     return rc;