Termios2 is a modern Linux feature thats allows usage of any custom baudrate instead of hardcoded values from the 1970's. Also, this patch improves compatibility with the win32 api, because windows allow any custom baudrates by design.
Example software for which this patch is needed: http://www.vi-soft.com.ua/index_e.htm
This software for reading/writing/patch fullflash of Siemens mobile phones, where serial speed is important. V-Klay uses 1600000 baudrate and perfectly works on Linux with termios2 and Prolific PL2303.
-- v4: ntdll/unix: Use termios2 for serial when possible.