Avoid embedding struct mbuf in other structures

struct mbuf uses a C99 open char array to allow inlining data. Inlining
this in another structure is however a GNU extension. The inlines used
so far in struct Slirp were actually only needed as head of struct
mbuf lists. This replaces these inline with mere struct quehead,
and use casts as appropriate.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>