module: Post-merge tasks for dirauth modularization

We've identified a series of things to do post-merge. They aren't that big and should be quite straightforward to achieve (no need to do them in that order):

1. Splitting up dirvote_common.

nickm and I have discussed this and the decision is:

• voting_schedule.{c|h} for the dirvote_common.c stuff
• shared_random_client.{c|h} for shared_random_common.c.

2. Adding more #ifdefs (nickm)

3. Rename and move dirvote_get_voter_sig_by_alg()

We'll move this to networkstatus.c and namespace is networkstatus_

4. Make dirvote_parse_sr_commits take a const ptr for tokens.

For this to happen, we need to refactor the code so find_opt_by_keyword() can take a const pointer.

5. Add a changes file.