More correct label for the save page menu item

Use "Save page as…" since the item will open a save panel.
BMP sniffing: check more bytes

* Check that the two high-order byte of the DIB header size are 0 (the
largest known size according to wikipedia is 124 bytes).
* This avoids identifying any text file that starts with "BM" as a BMP
file.

Fixes #10912.