dip1000 and preview in combine to cause extra safety errors

[Walter Bright]

The point of @safe by default for C declarations was:

1. so that we would not be deluged with complaints about breaking existing code

2. so people would use it

What people *will* do with C unsafe by default is:

1. slap `@trusted:` at the beginning and go on their merry way, and nothing was 
accomplished except annoying people