all: Add package runtimeos for runtime.GOOS comparisons
I grew tired of hand written string comparisons. This adds generated
constants for the GOOS values, and predefined Is$OS constants that can
be iffed on. In a couple of places I rewrote trivial switch:es to if:s,
and added Illumos where we checked for Solaris (because they are
effectively the same, and if we're going to target one of them that
would be Illumos...).
This removes the special handling of minor versions as major when the
actual major is zero, and adds the special case that upgrades from 0.x
to 1.x are considered minor. 0.x to 2.x or 1.x to 2.x etc are still
considered major.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4226
