rewrite raw SYS_socketcall usages

This is not completely perfect; the bits for allocating remote memory for socketcall args need to be radically rewritten both to make them simpler to understand and to work when we don't have a definition of `socketcall_args` available, for instance.  I think there are also some host/target distinctions that need to be made.  But this is progress, at least: no more `SYS_socketcall` usages in the main code.