debian/: Update for Debhelper 10.

Possibly controversially: I've forced the libdir back to `/usr/lib',
without an architecture-specific subdirectory.  This makes it easy for
architecture-independent packages to drop things into appropriate
subdirectories, while still /permitting/ architecture-dependent services
and similar.

Relatedly, though, move the `pkg-config' dropping into `/usr/share' by
default.