Add support for multiarch.

Add support for multiarch.

Open compat level 9, which incompatibly changes dh_auto_configure behavior
to set --libdir and --libexecdir to the multiarch directory path.  This
requires dpkg-dev 1.16.0 (not yet released) for the multiarch directory
variable, so bump the dependency to this version.

Also set a new substvar, misc:Pre-Depends, to multiarch-support, a virtual
package provided by versions of eglibc that support the multiarch library
paths at runtime; this needs to be a pre-dependency to ensure unpacked but
not-yet-configured libraries can still be found during upgrades, so library
packages converting to multiarch (i.e., switching to compat 9) will need to
add this substitution by hand to debian/control.