The TCSETS2 uses the more modern ioctl that supports setting custom
bitrates. The only loss here is the ability to specify
`optional_actions`; compare
https://fossies.org/dox/glibc-2.25/sysdeps_2unix_2bsd_2tcsetattr_8c_source.html

As these are not used anyway, this is a net gain in functionality.

Gated through conditional compilation to run only on Linux.

With support of the `TCSETS2` ioctl, custom bitrates are now possible on
Linux. This commit adds support for these.