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...).
Prompted by https://forum.syncthing.net/t/infinite-filesystem-recursion-detected/15285. In my opinion the filesystem shouldn't throw warnings but pass on errors for the caller to decide what's to be happening with it. Right now in this PR an infinite recursion is a normal scan error, i.e. folder is in failed state and displays failed items, but no warning. I think that's appropriate but if deemed appropriate an additional warning can be thrown in the scanner.
