Ruby: process and thread lifecycle hooks.

This feature allows one to specify blocks of code that are called when certain
lifecycle events occur.  A user configures a "hooks" property on the app
configuration that points to a script.  This script will be evaluated on boot
and should contain blocks of code that will be called on specific events.

An example of configuration:

{
    "type": "ruby",
    "processes": 2,
    "threads": 2,
    "user": "vagrant",
    "group": "vagrant",
    "script": "config.ru",
    "hooks": "hooks.rb",
    "working_directory": "/home/vagrant/unit/rbhooks",
    "environment": {
        "GEM_HOME": "/home/vagrant/.ruby"
    }
}

An example of a valid "hooks.rb" file follows:

File.write("./hooks.#{Process.pid}", "hooks evaluated")

on_worker_boot do
    File.write("./worker_boot.#{Process.pid}", "worker booted")
end

on_thread_boot do
    File.write("./thread_boot.#{Process.pid}.#{Thread.current.object_id}",
               "thread booted")
end

on_thread_shutdown do
    File.write("./thread_shutdown.#{Process.pid}.#{Thread.current.object_id}",
               "thread shutdown")
end

on_worker_shutdown do
    File.write("./worker_shutdown.#{Process.pid}", "worker shutdown")
end

This closes issue #535 on GitHub.

src/nxt_application.h

@@ -74,6 +74,7 @@ typedef struct {
 typedef struct {
     nxt_str_t  script;
     uint32_t   threads;
+    nxt_str_t  hooks;
 } nxt_ruby_app_conf_t;