Add more people to AUTHORS

From a suggestion of Anders:

> Actually, since it’s probably best to keep them in an order that doesn’t
> change over time, here are the authors sorted by when their first commit
> was merged:

$ git log origin/master --pretty='%an' --topo-order --reverse | awk '{if (!h[$0]) {print $0; h[$0]=1}}'

I removed duplicates and most of the middle initials.
Fix spelling errors in AUTHORS (caught by ispell)
Add tests to detect new_variable_* overriding previously set values

[davidben@mit.edu: Fix some nits]
Make the fullvar variable test more reasonable

Calling set on the output of get really should a no-op. Also test that
the new version of the setters were installed.
Don't g_value_unset an uninitialized GValue

Silences a GObject assertion.
When replacing a variable, keep the old value

This unbreaks variable behavior on module reload.

Reported-By: Robert Jacobs <rnjacobs@mit.edu>
Add a .mailmap to canonicalize commit names/emails
Update and make configurable the default format

- Rather than http://zephyr.1ts.org/wiki/df, use http://mit.edu/df, which is
  shorter
- Rather than a bare URL, prefix "Config error: see " to make it clearer what
  is going on
- Make the default format configurable using ./configure, to aid users with
  particularly severe length constraints