Introducting application graceful stop.  For now only used when application
process reach request limit value.

This closes #585 issue on GitHub.

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.

If an exception was raised with a backtrace of zero length, the
nxt_ruby_exception_log() routine would return without logging the
exception class and message.  This commit fixes the issue.

Ruby 3.0 deprecated rb_cData with the intention to remove it in release 3.1.
This commit changes references of rb_cData to rb_cObject.  This was done so we
can support distributions that package Ruby 3.0, such as Fedora 34.

We also need to call rb_undef_alloc_func because we're no longer deriving from
rb_cData.  This prevents unnecessary allocations.

See:
https://docs.ruby-lang.org/en/3.0.0/doc/extension_rdoc.html

"It is recommended that klass derives from a special class called Data
(rb_cData) but not from Object or other ordinal classes.  If it doesn't,
you have to call rb_undef_alloc_func(klass)."

The Ruby interpreter expects an explicit setlocale() call before initialization
to pick up character encodings in the "Encoding" class from the environment.

This closes #531 issue on GitHub.

Ruby threads need to be created with GVL; otherwise, an attempt to access
locked resources may occur, causing a crash.

The issue was occasionally reproduced on Ubuntu 18.04 with Ruby 2.5.1
while running test_ruby_application_threads.

Application terminates in case of thread creation failure.

This closes #482 issue on GitHub.

This shall save a couple of CPU cycles in request processing.

Since the introduction of rootfs feature, some language modules
can't be configured multiple times.

Now the configure generates a separate nxt_<module>_mounts.h for
each module compiled.