Add IoctlGetTermios and IoctlGetTermios to retreive and manipulate
Termios structures on Linux, akin to the already existing implementation
or Solaris.

If desired, functions conforming to POSIX tcgetattr/tcsetattr could be
implemented on top of these.

Change-Id: I2a71061bad2e632c597b6f6184ad6374c46a43ee
Reviewed-on: https://go-review.googlesource.com/47330Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>