On Unix support using a system-installed jsvc rather than the embed…

…ded one through the `capsule.daemon.jsvc` property (#6)

README.md

@@ -44,6 +44,7 @@ The following additional manifest entries and system properties can be used to c
      - `capsule.daemon.checkOnly`: `jsvc` check run, won't start the service.
      - `capsule.daemon.debug`: turn on debug `jsvc` logging.
      - `capsule.daemon.verbose`: turn on verbose `jsvc` logging.
+     - `capsule.daemon.jsvc`: specifies the pathname of a system-installed `jsvc` command to be used instead of the one provided by `capsule-daemon`.
    - Manifest entries:
      - `Init-Class`: class containing the `init` method (default: none).
      - `Init-Method`: static `String[] -> String[]` service initialization method, it will be run as `root`; the return value will be passed to the `Start` method (default: none).

src/main/java/DaemonCapsule.java

@@ -90,6 +90,7 @@ public class DaemonCapsule extends Capsule {
     private static final String PROP_CHECK_ONLY = "capsule.daemon.checkOnly";
     private static final String PROP_DEBUG = "capsule.daemon.debug";
     private static final String PROP_VERBOSE = "capsule.daemon.verbose";
+    private static final String PROP_JSVC = "capsule.daemon.jsvc";
 
     private static final Map.Entry<String, String> ATTR_INIT_CLASS = ATTRIBUTE("Init-Class", T_STRING(), null, true, "Class containing the init method (default: none, Unix only)");
     private static final Map.Entry<String, String> ATTR_INIT_METHOD = ATTRIBUTE("Init-Method", T_STRING(), null, true, "Static 'String[] -> String[]' service initialization method short name run as 'root'; the return value will be passed to the 'Start' method (default: none, Unix only)");
@@ -115,8 +116,14 @@ public DaemonCapsule(Capsule pred) {
 
     @Override
     protected Path getJavaExecutable() {
-        if (svcExec == null)
+        if (svcExec == null) {
+            if (isUnix()) {
+                final String systemJsvc = getProperty(PROP_JSVC);
+                if (systemJsvc != null)
+                    return (svcExec = Paths.get(systemJsvc));
+            }
             svcExec = setupBinDir().resolve(platformExecPath()).toAbsolutePath().normalize();
+        }
         return svcExec;
     }