trim: Enable dependency analysis (--auto) by default.

The --exact mode of trimming can be useful, but only if the user has
somehow performed all the dependency analysis needed manually, (such
as by extracting a list of calls from a previous run by
--print-callset). Since --exact inherently requires more care to be
used, it makes sense to also require an extra command-line option, and
to thus prefer the easier-to-use --auto mode by default.