A thread has-a selected/current frame

[Andrew Cagney <ac131313@redhat.com>]

Hello,

At present the selected and current frame are maintained as single 
globals, and independant of the selected and current thread.  As a 
consequence, each time GDB switches between threads, all global frame 
state is flushed.

I'm intending to add a "current frame" and "selected frame" to 
"thread_info" so that, instead this can be maintained on a per-thread basis.

The tricky one is the sentinel-frame.  It needs to be able to perform an 
arbitrary projection of the virtual thread's register onto the target 
below's physical threads and memory.   I think this suggests a 
sentinel-frame parameterized by target, and not just the current regcache.

Thoughts?
Andrew