net/http/cgi: merge upstream changes to default env vars

Incorporate upstream modifications to the cgi package's set of rules
about which environment variables should be inherited by child processes
by default on each platform. In particular this permits tests to pass on
NetBSD by preserving the value of the LD_LIBRARY_PATH environment
variable.

This is a partial backport of the following upstream CLs:

    https://golang.org/cl/263802
    https://golang.org/cl/263577
    https://golang.org/cl/254740

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/264097

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-fa66bd11bbe58943e273cfa74356771c996f5b24
+28f3df468666787f83f94220312383a7c267a8ce
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/net/http/cgi/host.go

@@ -37,15 +37,15 @@ var trailingPort = regexp.MustCompile(`:([0-9]+)$`)
 
 var osDefaultInheritEnv = func() []string {
 	switch runtime.GOOS {
-	case "darwin":
+	case "darwin", "ios":
 		return []string{"DYLD_LIBRARY_PATH"}
-	case "linux", "freebsd", "openbsd":
+	case "linux", "freebsd", "netbsd", "openbsd":
 		return []string{"LD_LIBRARY_PATH"}
 	case "hpux":
 		return []string{"LD_LIBRARY_PATH", "SHLIB_PATH"}
 	case "irix":
 		return []string{"LD_LIBRARY_PATH", "LD_LIBRARYN32_PATH", "LD_LIBRARY64_PATH"}
-	case "solaris":
+	case "illumos", "solaris":
 		return []string{"LD_LIBRARY_PATH", "LD_LIBRARY_PATH_32", "LD_LIBRARY_PATH_64"}
 	case "windows":
 		return []string{"SystemRoot", "COMSPEC", "PATHEXT", "WINDIR"}