[rfa] Add bfd-in-memory io vector

[Andrew Cagney]

>>> I've tested this on i386 GNU/Linux with no regressions.  However, given 
>>> my experience with the previous IOVEC change, I'm not sure how 
>>> significant that result is :-/
> 
> 
> A significant user of the BIM code is Cygwin's DLL stuff, both
> creating DLLs (with ld -shared) and linking against them (directly,
> not with import libraries).

s/A significant/The evil/, as I wrote:

> Note that it doesn't completly consolidate code using "struct bfd_in_memory" as some of it is pretty evil (grep for remaining references to BFD_IN_MEMORY).  I'll do another pass at eliminating more of the BFD_IN_MEMORY references in a follow-up patch. 

I tried to leave that code be.  Not sure how successful I was.

Andrew