*-user: plugin syscalls

*-user: plugin syscalls

To avoid too much duplication add a wrapper that the existing trace
and the new plugin calls can live in. We could move the -strace code
here as well but that is left for a future series as the code is
subtly different between the bsd and linux.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[AJB: wrap in syscall-trace.h, expand commit msg]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>