all: Add build constants for runtime.GOOS comparisons (#8442)

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...).

lib/build/runtimeos.gen.go

@@ -0,0 +1,38 @@
+// Code generated by runtimeos.sh. DO NOT EDIT.
+package build
+
+import "runtime"
+
+const (
+	AIX       = "aix"
+	Android   = "android"
+	Darwin    = "darwin"
+	Dragonfly = "dragonfly"
+	FreeBSD   = "freebsd"
+	Illumos   = "illumos"
+	IOS       = "ios"
+	JS        = "js"
+	Linux     = "linux"
+	NetBSD    = "netbsd"
+	OpenBSD   = "openbsd"
+	Plan9     = "plan9"
+	Solaris   = "solaris"
+	Windows   = "windows"
+)
+
+const (
+	IsAIX       = runtime.GOOS == AIX
+	IsAndroid   = runtime.GOOS == Android
+	IsDarwin    = runtime.GOOS == Darwin
+	IsDragonfly = runtime.GOOS == Dragonfly
+	IsFreeBSD   = runtime.GOOS == FreeBSD
+	IsIllumos   = runtime.GOOS == Illumos
+	IsIOS       = runtime.GOOS == IOS
+	IsJS        = runtime.GOOS == JS
+	IsLinux     = runtime.GOOS == Linux
+	IsNetBSD    = runtime.GOOS == NetBSD
+	IsOpenBSD   = runtime.GOOS == OpenBSD
+	IsPlan9     = runtime.GOOS == Plan9
+	IsSolaris   = runtime.GOOS == Solaris
+	IsWindows   = runtime.GOOS == Windows
+)

lib/build/runtimeos.sh

@@ -0,0 +1,43 @@
+# Copyright (C) 2022 The Syncthing Authors.
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at https://mozilla.org/MPL/2.0/.
+
+#!/bin/sh
+
+# List known operating system/architecture combos, removing the
+# architecture, and making constants of them.
+
+osname() {
+    echo "$1" | awk '{print toupper(substr($1, 1, 1)) substr($1, 2)}' \
+        | sed s/aix/AIX/i \
+        | sed s/bsd/BSD/i \
+        | sed s/ios/IOS/i \
+        | sed s/js/JS/i
+}
+
+osconstants() {
+    echo "// Code generated by runtimeos.sh. DO NOT EDIT."
+    echo "package build"
+    echo "import \"runtime\""
+
+    echo "const ("
+    for OS in $(go tool dist list | sed 's|/.*||' | sort | uniq) ; do
+        name=$(osname "$OS")
+        echo "$name = \"$OS\""
+    done
+    echo ")"
+    echo
+
+    echo "const ("
+    for OS in $(go tool dist list | sed 's|/.*||' | sort | uniq) ; do
+        name=$(osname "$OS")
+        echo "Is$name = runtime.GOOS == $name"
+    done
+    echo ")"
+}
+
+osconstants > runtimeos.gen.go
+
+gofmt -w -s runtimeos.gen.go