model: rework assert_bug() and assert_user_bug() interfaces

model: rework assert_bug() and assert_user_bug() interfaces

We don't need assert_bug_immediate(), and we don't need some of the
optional parameters in assert_bug(). Just simplify down to two
interfaces:

    assert_bug()
    assert_user_bug()

The former is useful for all bugs triggered from model-checker threads,
and the latter is useful from a user-thread context, so that you can
immediately switch to the model-checking context and exit the program -
but *only if* the trace is currently-feasible.