c-user: Move "POSIX Initialization Thread Configuration"

Update #3836.

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+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+POSIX Initialization Thread Configuration
+=========================================
+
+The ``<rtems/confdefs.h>`` configuration system can automatically generate a
+POSIX Initialization Threads Table named ``POSIX_Initialization_threads`` with
+a single entry.  The following parameters control the generation of that table.
+
+.. index:: CONFIGURE_POSIX_INIT_THREAD_ENTRY_POINT
+
+.. _CONFIGURE_POSIX_INIT_THREAD_ENTRY_POINT:
+
+CONFIGURE_POSIX_INIT_THREAD_ENTRY_POINT
+---------------------------------------
+
+CONSTANT:
+    ``CONFIGURE_POSIX_INIT_THREAD_ENTRY_POINT``
+
+DATA TYPE:
+    POSIX thread function pointer (``void *(*entry_point)(void *)``).
+
+RANGE:
+    Undefined or a valid POSIX thread function pointer.
+
+DEFAULT VALUE:
+    The default value is ``POSIX_Init``.
+
+DESCRIPTION:
+    ``CONFIGURE_POSIX_INIT_THREAD_ENTRY_POINT`` is the entry point
+    (a.k.a. function name) of the single initialization thread defined by the
+    POSIX API Initialization Threads Table.
+
+NOTES:
+    The user must implement the function ``POSIX_Init`` or the function name
+    provided in this configuration parameter.
+
+.. index:: CONFIGURE_POSIX_INIT_THREAD_STACK_SIZE
+
+.. _CONFIGURE_POSIX_INIT_THREAD_STACK_SIZE:
+
+CONFIGURE_POSIX_INIT_THREAD_STACK_SIZE
+--------------------------------------
+
+CONSTANT:
+    ``CONFIGURE_POSIX_INIT_THREAD_STACK_SIZE``
+
+DATA TYPE:
+    Unsigned integer (``size_t``).
+
+RANGE:
+    Zero or positive.
+
+DEFAULT VALUE:
+    The default value is 2 \* RTEMS_MINIMUM_STACK_SIZE.
+
+DESCRIPTION:
+    ``CONFIGURE_POSIX_INIT_THREAD_STACK_SIZE`` is the stack size of the single
+    initialization thread defined by the POSIX API Initialization Threads
+    Table.
+
+NOTES:
+    If the stack size specified is greater than the configured minimum, it must
+    be accounted for in ``CONFIGURE_EXTRA_TASK_STACKS``.  See :ref:`Reserve
+    Task/Thread Stack Memory Above Minimum` for more information about
+    ``CONFIGURE_EXTRA_TASK_STACKS``.
+
+.. index:: CONFIGURE_POSIX_INIT_THREAD_TABLE
+
+.. _CONFIGURE_POSIX_INIT_THREAD_TABLE:
+
+CONFIGURE_POSIX_INIT_THREAD_TABLE
+---------------------------------
+
+CONSTANT:
+    ``CONFIGURE_POSIX_INIT_THREAD_TABLE``
+
+DATA TYPE:
+    Boolean feature macro.
+
+RANGE:
+    Defined or undefined.
+
+DEFAULT VALUE:
+    This field is not defined by default, as the user MUST select their own API
+    for initialization tasks.
+
+DESCRIPTION:
+    ``CONFIGURE_POSIX_INIT_THREAD_TABLE`` is defined if the user wishes to use
+    a POSIX API Initialization Threads Table.  The table built by
+    ``<rtems/confdefs.h>`` specifies the parameters for a single thread. This
+    is sufficient for applications which initialization the system from a
+    single task.
+
+    By default, this field is not defined as the user MUST select their own API
+    for initialization tasks.
+
+NOTES:
+    The application may choose to use the initialization tasks or threads table
+    from another API.
+
+    A compile time error will be generated if the user does not configure any
+    initialization tasks or threads.