MINOR: rawsock: set connection error codes when returning from recv/send/splice

MINOR: rawsock: set connection error codes when returning from recv/send/splice

For a long time the errno values returned by recv/send/splice() were not
translated to connection error codes. There are not that many eligible
and having them would help a lot when debugging some complex issues where
logs disagree with network traces. Let's add them now.

(cherry picked from commit 822d82caf4165f0f6da681737c7e3db17d01f599)
Signed-off-by: Willy Tarreau <w@1wt.eu>