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.

The process abstraction has changed to:

  setup(task, process)
  start(task, process_data)
  prefork(task, process, mp)

The prefork() occurs in the main process right before fork.

The file src/nxt_main_process.c is completely free of process
specific logic.

The creation of a process now supports a PROCESS_CREATED state.  The
The setup() function of each process can set its state to either
created or ready.  If created, a MSG_PROCESS_CREATED is sent to main
process, where external setup can be done (required for rootfs under
container).

The core processes (discovery, controller and router) doesn't need
external setup, then they all proceeds to their start() function
straight away.

In the case of applications, the load of the module happens at the
process setup() time and The module's init() function has changed
to be the start() of the process.

The module API has changed to:

  setup(task, process, conf)
  start(task, data)

As a direct benefit of the PROCESS_CREATED message, the clone(2) of
processes using pid namespaces now doesn't need to create a pipe
to make the child block until parent setup uid/gid mappings nor it
needs to receive the child pid.

Ruby and Java modules now use this function instead of own
implementations.

This closes #371 issue on GitHub.

There was a change (ruby/ruby@6c70fed) in Ruby 2.6 that moved
RUBY_DESCRIPTION global constant definition out of Init_version().
Unit initialized Ruby incorrectly, so the constant was not defined.

This closes #330 issue on GitHub.

This closes #223 issue on GitHub.