build: change vf_dlopen test

Didn't work on Windows. Apparently, WIN32 is not set in the Makefile.
stream: remove some more forgotten network stuff

Was not cleanly removed with internal network code removal.
m_struct: try to be more standard C

The main problem is that this m_struct stuff uses pointers for offsets
(why...), so we mangle it by intptr_t. This stuff really should use ints
(or in theory ptrdiff_t) for offsets, but changing it would be too much
effort, and hopefully this m_struct stuff will go away and replaced by
the common option parser mechanism instead.

Shuts up warnings on Windows.

Patch suggested by jon_y and rossy on IRC.