Support %ENV.NAME% substitutions.

These can appear anywhere %APPDIR% and %VERSION% substitutions occur (pretty
much everywhere in getdown.txt).

In this case the NAME is the name of an environment variable, and the value of
the environment variable is substituted into the text.

src/main/java/com/threerings/getdown/data/Application.java

@@ -1090,6 +1090,21 @@ public class Application
     {
         arg = arg.replace("%APPDIR%", _appdir.getAbsolutePath());
         arg = arg.replace("%VERSION%", String.valueOf(_version));
+
+        // if this argument contains %ENV.FOO% replace those with the associated values looked up
+        // from the environment
+        if (arg.contains(ENV_VAR_PREFIX)) {
+            StringBuffer sb = new StringBuffer();
+            Matcher matcher = ENV_VAR_PATTERN.matcher(arg);
+            while (matcher.find()) {
+                String varName = matcher.group(1), varValue = System.getenv(varName);
+                if (varName == null) varName = "MISSING:" + varName;
+                matcher.appendReplacement(sb, varValue);
+            }
+            matcher.appendTail(sb);
+            arg = sb.toString();
+        }
+
         return arg;
     }
 
@@ -1783,4 +1798,7 @@ public class Application
     protected FileChannel _lockChannel;
 
     protected static final String[] SA_PROTO = ArrayUtil.EMPTY_STRING;
+
+    protected static final String ENV_VAR_PREFIX = "%ENV.";
+    protected static final Pattern ENV_VAR_PATTERN = Pattern.compile("%ENV\\.(.*?)%");
 }