[RFC] Register sets

[Andrew Cagney]

> The read_regset function is necessary to support the `gcore' command.
> It should use regcache_raw_read to fetch all relevant registers from
> the running target, such that we don't need target_fetch_registers(-1)
> first.

GDB currently has a conflict here.  GCORE should definitly use 
regcache_raw_read, but that still leaves the ptrace code writing back 
register values - it uses regcache_collect.  Hmm, perhaphs it two should 
be using a regcache_raw_read anyway - might even make it possible to 
eliminate "target_prepare_to_store"?

I like the use of BFD's names.

Andrew