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_conf_validation.c

@@ -732,6 +732,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_ruby_members[] = {
         .name       = nxt_string("threads"),
         .type       = NXT_CONF_VLDT_INTEGER,
         .validator  = nxt_conf_vldt_threads,
+    }, {
+        .name       = nxt_string("hooks"),
+        .type       = NXT_CONF_VLDT_STRING
     },
 
     NXT_CONF_VLDT_NEXT(nxt_conf_vldt_common_members)