[v6] multi-executable support

[Pedro Alves <pedro at codesourcery dot com>]

Here's my current patch to add multi-executable support to GDB.
Following the last discussion, 

 http://sourceware.org/ml/gdb-patches/2009-09/msg00194.html

This makes it so that there's always an inferior, and the
"symbol-spaces" are relegated to the background as an implementation
detail, renamed as program spaces.

This is almost the same as the patch I had posted on the link
above as "v5", with s/symbol-spaces/program-spaces/, generally
cleaned up, and documentation updated to reflect the simpler
model and to address other points Eli raised previously.

I'm hoping this isn't far from putting in, as it's been
a nuisance getting this patch close to mainline, as it
tends to regress easily due to new code going in that
doesn't consider multi-exec.

Eli, could you please take a new look at the docs bits?
I may have missed something, but I hope nothing big.

Note: the changeLogs are a bit stale and needs a
new update.

No regressions on x86_64-unknown-linux-gnu.

-- 
Pedro Alves

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