indent: check for bad usernames and emails

indent: check for bad usernames and emails

For compiling the list of contributors for each release, we rely on
useful names and email addresses. We also regularly generate statistics
about number of contributors, which requires identifying unique authors.

This PR does the following:
1. Enforce in `indent` that users have a sensible name set that consists
of at least two words (git config --global user.name)
2. Enforce in `indent` that users have sensible email addresses set by
blocking some common invalid ones.
3. Add a .mailmap file that corrects a collection of invalid
names/addresses already part of the history. This is a standard git
file, which will replace output of `git log` and similar commands.

Step 3 is necessary to make ./indent pass on master.