package version and variant info in `main.log`

[Werner LEMBERG]

I've noticed that in `main.log` files there is no information what
version and variants have been requested – for example,

  ImageMagick @6.9.9-40_7+x11

Wouldn't it make sense to make `port` add this information to the very
top of `main.log`? This would also help identify various `make.log` files
more easily.


    Werner