[m-rev.] for review: extend modes of operation

[Zoltan Somogyi]

For review by Julien, since this extends his work.

The two parts I am most interested in feedback on are
the exceptions in decide_op_mode (are they ok? do we need
more exceptions?), and whether it is ok that we now assume
that rebuilding the option set should NOT change the mode
of operation.

Also, should the op_mode type itself be in globals.m, with
decide_op_mode in handle_options.m? The op_mode is used
only in mercury_compile.m (now), but the latter would allow the
deletion of the (slightly incorrect) redundant computation of Link,
and it requires the former as a prerequisite.

Zoltan.
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