gsocket: add G_IO_ERROR_CONNECTION_CLOSED

Add G_IO_ERROR_CONNECTION_CLOSED as an alias for
G_IO_ERROR_BROKEN_PIPE, and also return it on ECONNRESET.

It doesn't really make sense to try to distinguish EPIPE and
ECONNRESET at the GLib level, since the exact choice of which error
gets returned in what conditions depends on the OS. Given that, we
ought to map the two errors to the same value, and since we're already
mapping EPIPE to G_IO_ERROR_BROKEN_PIPE, we need to map ECONNRESET to
that too. But the existing name doesn't really make sense for sockets,
so we add a new name.

https://bugzilla.gnome.org/show_bug.cgi?id=728928