Resolve the current user home even when $HOME is not set

* Fixes oracle/truffleruby#2784
* Use getpwuid_r(getuid()) for simplicity and reliability.

core/dir/home_spec.rb

@@ -20,6 +20,18 @@
       Dir.home.should_not.frozen?
     end
 
+    it "returns a string with the filesystem encoding" do
+      Dir.home.encoding.should == Encoding.find("filesystem")
+    end
+
+    platform_is_not :windows do
+      it "works even if HOME is unset" do
+        ENV.delete('HOME')
+        Dir.home.should.start_with?('/')
+        Dir.home.encoding.should == Encoding.find("filesystem")
+      end
+    end
+
     platform_is :windows do
       ruby_version_is "3.0" do
         it "returns the home directory with forward slashs and as UTF-8" do
@@ -65,6 +77,10 @@
     it "returns a non-frozen string" do
       Dir.home(ENV['USER']).should_not.frozen?
     end
+
+    it "returns a string with the filesystem encoding" do
+      Dir.home(ENV['USER']).encoding.should == Encoding.find("filesystem")
+    end
   end
 
   it "raises an ArgumentError if the named user doesn't exist" do