Print unnamed Dummy symbols more prominently

Previously, they were just printed as _12 or pretty printed as just 12, which
looked like a number in an expression.  This way, if a Dummy leaks out, it is
clear.
Don't use unnamed Dummy objects in doctests

I changed _mask_nc to accept a name argument to allow this.  The doctest in
the Dummy docstring was skipped.

Resetting Dummy._count not only make the doctest confusing, but it's a bad idea
and could potentially lead to failures as some internal code could still have
Dummy objects laying around that would now compare equal to new, unrelated
ones.
append number to name of _mask_nc Dummy symbols

If no name is given, use 'mask'.
Merge pull request #2138 from smichr/unnamed_dummy

Print unnamed Dummy symbols more prominently