Work around inconsistent version numbering in 0.9.8f (release).

The version code of the release should have been 09086f (6=f, f=release)
but accidently it was marked "090870" (which would be "0.9.8g-dev").

Therefore we now use "090871" for the development of 0.9.8g. Once
0.9.8g is released, the problem will be "healed". We have never done
beta releases for 0.9.x-stable patch releases, so 090871 would never
be used in practice.

PR: #1589