module system: add httpserver module unless explicitly removed

Fixes #357.

This patch adds go config.json a "default" list, a list of modules that
are always added to every built image, unless explicitly subtracted with
the syntax "-name".

This patch puts "httpserver" in the new "default" list. So now, when
a user does "make image=memcached", he will get an image with both
memcached and httpserver, as if "make image=memcached,httpserver" was
used. To get only memcached, without httpserver, one can do

   make image=memcached,-httpserver

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
test/misc-malloc: fix data corruption in worker thread

We capture len and tf by reference, even though they are stack variables
that immediately go out of scope.

Change to capture by value. Fixes #370.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>