[fink-core] changes needed to matching kernel version with macOS release

[Hanspeter N. <fi...@sn...>]

BigSur11.1 broke the kernel version to macOS release mapping that 
existed through the entire 10.X series.

Previously, the major kernel version tracked the minor macOS release 
(with a -4 offset). So macOS 10.13 is darwin17, 10.15 is darwin19, etc. 
The minor kernel version mostly tracked point updates for a particular 
macOS (darwin19.5.0 was 10.15.5, but darwin19.5.6 is both 10.15.6 and 
10.15.7).

BigSur 11.0 is darwin20.1.0. So the -4 offset no longer held and we 
added a conditional to use a -20 offset when translating the kernel 
version to the macOS release.

But BigSur 11.1 is darwin20.2.0. Apple is now using the kernel minor 
version to track macOS minor versions. And macOS minor versions are more 
frequent than in the past. In the 10.X series, they came annually (High 
Sierra, Mojave, Catalina, etc). Now they're coming ~ monthly.

We're not going to be able to keep making new dists and releases with 
every BigSur minor release. Apple seems to be thinking that BigSur minor 
releases are like 10.X point releases (11.2 is really like 10.16.2).

I'm testing a solution that adds a new function 
Services.pm::get_kernel_vers_minor() that returns the kernel minor 
version, and then instead of mapping darwin20 to 11.(kernel_vers-20), we 
do 11.(kernel_vers_minor-1).
This seems to rescue an existing fink install from 11.0 that got 
upgraded to 11.1, but bootstrap fails.

Thoughts?

Hanspeter