Inferior function call command set

[Doug Evans]

Andrew Cagney writes:
 > GDB has a number of commands for controlling the behavior of inferior 
 > function calls (developers think of them as call dummies).  Looking in 
 > "infcall.c" I find:
 > 
 > 	set/show coerce-float-to-double yes|no
 > 	set/show unwindonsignal yes|no
 > 
 > And I'm now looking to add a third:
 > 
 > 	set/show call-location on-stack|at-start|auto

Questions:

Out of curiousity, is there any need to have a runtime choice?

What happens if a value is set that the target doesn't support?
I presume this will be flagged as an error, right?

Is there any target that actually supports more than one?
(and that has all forms consistently working, rather than the
usual one working and the others bitrotting away ...)