apt-get dist-upgrade falied with a dbus trigger loop due to an unconfigured library (libexpat1)

While attempting to upgrade from before the gcc-5 transition to after it apt-get dist-upgrade exploded in the middle. It was attempting to handle dbus triggers but was unable to do so due to an unconfigured library, leading to a loop attempting to run the triggers and to the install failing in the middle:

  $ sudo apt-get dist-upgrade
  [...]
  Preparing to unpack .../libexpat1_2.1.0-7_amd64.deb ...
  De-configuring libexpat1:i386 (2.1.0-6ubuntu1) ...
  Unpacking libexpat1:amd64 (2.1.0-7) over (2.1.0-6ubuntu1) ...
  Preparing to unpack .../libexpat1_2.1.0-7_i386.deb ...
  Unpacking libexpat1:i386 (2.1.0-7) over (2.1.0-6ubuntu1) ...
  [...]
  Preparing to unpack .../systemd-sysv_224-1ubuntu3_amd64.deb ...
  Unpacking systemd-sysv (224-1ubuntu3) over (224-1ubuntu1) ...
  Processing triggers for man-db (2.7.0.2-5) ...
  dpkg: dependency problems prevent processing triggers for dbus:
   dbus depends on libexpat1 (>= 2.0.1); however:
    Package libexpat1:amd64 is not configured yet.

  dpkg: error processing package dbus (--configure):
   dependency problems - leaving triggers unprocessed
  dpkg: dependency problems prevent processing triggers for dbus:
   dbus depends on libexpat1 (>= 2.0.1); however:
    Package libexpat1:amd64 is not configured yet.
  [...]
  dpkg: too many errors, stopping
  Errors were encountered while processing:
   dbus
   dbus
  [...]
   dbus
   dbus
  Processing was halted because there were too many errors.

An apt-get install -f was unable to make progress from the resulting situation simply repeating the dbus trigger loop.

This was simply correctable (not simple to figure out, thank you cjwatson) by asking dpkg to configure libexpat1:

  $ sudo dpkg --configure libexpat1

From there a sudo apt-get install -f, and then a sudo apt-get install ubuntu-desktop^ (mine was a desktop install) sorted me out.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: apt 1.0.9.10ubuntu6
ProcVersionSignature: Ubuntu 4.1.0-3.3-generic 4.1.3
Uname: Linux 4.1.0-3-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.18-0ubuntu7
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Aug 18 11:21:45 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-06-20 (788 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Alpha amd64 (20130618)
SourcePackage: apt
UpgradeStatus: Upgraded to wily on 2013-11-10 (645 days ago)