I got some reports that some targets have weird dso_schemes.

Therefore, I've added a sanity checker.

Note that it can be combined with almost any other argument (the other
arguments will be completely ignored), with "reconf" as the blatant
exception, since it also has the behavior of ignoring all following
command line arguments.  If --test-sanity and reconf are both used on
the command line, the first one wins.