sim,base: make checkpointMapIn warn if an unknown key is found

sim,base: make checkpointMapIn warn if an unknown key is found

The warning happens when a key is present in the checkpoint but not in the
values that gem5 source code knows about.

To do this, we must expose iteration over IniFile section keys. To not
have to make those classes public, a visitor method is implemented.

Change-Id: I23340a953f3e604642b97690a7328b10fdd740a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37575
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>