SmtEngine now fails with a ModalException if --incremental is not enabled

SmtEngine now fails with a ModalException if --incremental is not enabled
but a push/pop or multiple query is attempted (previously it could give
incorrect answers)

Also, fix some multi-query and push-pop tests that had wrong answers, and
support a new "% COMMAND-LINE: " gesture in regression tests so that a
test can pass additional, specific command line flags it wants to run
with (here, --incremental).

Also fix mkbuilddir script for when it's called from contrib/switch-config.