new DIP41: dmd/rdmd command line overhaul.

[timotheecour]

looks like no one read the DIP till the last section :)

I proposed there 2 ways to support both current flags and new 
flags, without possible confusion nor code breakage.

2a): dmd -flagstyle=new -of=blah ... (+ other stuff/deprecations 
taken into account)
2b): dmd2 (reminds of D2, ldc2, ldmd2), which behaves as dmd 
-flagstyle=new.

Either of those 2 options will break nothing, code that uses dmd 
-ofblah (etc) would continue to work. It's all described in DIP. 
Improvements are welcome.