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
Update .gitignore

Commit 353719a2ddec4d6a8458ee42af37514fedbd80bf adds gmarshal_funcs.c,
gmarshal_funcs.h.

Commit a223b6b426c1897977b987d0ddce434afba604da adds /bin/, barnowl,
tester and removes barnowl.bin, tester.bin.

Automake 1.13 (which enables the parallel testsuite harness by
default) adds runtests.sh.log, runtests.sh.trs, test-driver,
test-suite.log.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>