With this change, error messages include the offending characters or
name parts. Examples:
nul.txt: name is invalid, contains Windows reserved name: "nul"
foo>bar.txt: name is invalid, contains Windows reserved character: ">"
foo \bar.txt: name is invalid, must not end in space or period on Windows
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...).
