Some lines (incorrectly) had an indentation of 3 or 5, or 7 or 9,
or 11 or 13, or 15 or 17 spaces instead of 4, 8, 12, or 16.  Fix them.

Found with:

$ find src -type f | xargs grep -n '^   [^ ]';
$ find src -type f | xargs grep -n '^     [^ *]';
$ find src -type f | xargs grep -n '^       [^ ]';
$ find src -type f | xargs grep -n '^         [^ *]';
$ find src -type f | xargs grep -n '^           [^ +]';
$ find src -type f | xargs grep -n '^             [^ *+]';
$ find src -type f | xargs grep -n '^               [^ +]';
$ find src -type f | xargs grep -n '^                 [^ *+]';

Application handler can do anything with a stream object (including close it).
Once the stream is closed, Unit creates a new stream.

This closes #616 issue on GitHub.

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

This closes #585 issue on GitHub.

This closes #486 issue on GitHub.

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.

Some Perl compile options affects ABI and not using them while compiling
our module resulted in non-working build.

Notably on 32-bit Debian 10, Perl is built with -D_FILE_OFFSET_BITS=64
and our module after being compiled without this option caused segmentation
faults in unexpected places.