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...).
There was a logic mistake, so the limit in question wasn't used. On my
macOS this doesn't seem to matter, the hard limit returned is 2^63-1 and
setting the soft limit to that works. However I'm assuming that's not
the case for older macOSes since it was so nicely documented, so we
should still have this working. (10240 FDs should be enough for
anybody.)
This will decrease the risk of running out of file descriptors for the
database and other bad things, which could otherwise potentially happen
if we're serving lots of requests and scanning in parallel, etc.
Windows doesn't have a per process open file limit like Unix so we don't
need to worry about it there.
