linux-user: add SO_LINGER to {g,s}etsockopt

linux-user: add SO_LINGER to {g,s}etsockopt

Original implementation for setsockopt by Chen Gang[1]; all bugs mine,
including removing assignment for optname which hopefully makes the
logic easier to follow and moving some variables to make the code
more selfcontained.

[1] http://patchwork.ozlabs.org/patch/565659/

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Co-Authored-By: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180824085601.6259-1-carenas@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>