[PATCH v3 03/10] reg: Add system error printing function

[Jonathan Vollebregt]

The reg patches have been languishing on the list for a while now.

As Hugh's example showed format specifiers are the preferred way of 
doing rc strings, it just seems a shame we have to duplicate so much.

Can we have 107580 committed so I can rebase on it?

I still wish there were a better way to deal with the fixed-size 
reg_printfW output but it looks like there is none.