Invert default for -op, use -nop

[Frank Benoit]

The dmd option -op makes dmd not to strip the object path when
generating the object file. The default is to strip it.

This can lead to problems if one object file is overwritten from
another. E.g. you have to modules with the name 'mod.d' in different
directories/packages.

I think it would be better if the default chooses the way which does not
make problems. I know I can configure it in dmd.cfg. But this means,
first I have to trip into this pitfall.