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.
This is the support for "non standard baud rate". It means that any baud value can be accepted, it is not just an issue of one or some more values.