debian: Attempt to keep debian/changelog up-to-date.

debian: Attempt to keep debian/changelog up-to-date.

Invariably we forget to update the version number in debian/changelog as
we change OVS's own version number.  This is embarrassing.

This commit introduces two different times to automatically update the
debian/changelog version number: whenever boot.sh runs and whenever
"make dist" runs.  In the latter case, only the version number in the
distributed tarball is updated, but that seems OK.

Reported by Joan Cirer <joan@ev0.net> most recently, and by others over
the last year or so too.