Support for non-standard baud rate in Linux

libMonoPosixHelper does not support non standard baud rates on Linux.
I needed this feature as 3D printers often use 250000baud communications.

I would like to have this feature included in mainstream code, otherwise I have to download, patch and compile mono from the source code at each new release of mono.
It is a general purpose feature, it can be useful for many other applications.
I am confused; this will just accept any values, which seems wrong.

Instead, the setup_baud_rate should handle the new specific cases listed here.