Address several issues from compiling examples

Compiling the code from examples "code-block:: c" along with public
includes and a bsp exposed a few issues amongst a lot of false
positives. Address some of these:

* Terminate struct declarations with ';'. Mainly for pedantic
  correctness.

* Show ptrdiff_t instead of size_t for the sbrk() prototype, matching
  the current argument type in rtems.

* Replace some occurrences of unsigned16 with uint16_t.

* Fix odd type declaration, "uint8_t char*" -> "char*".

* Use appropriate helper to get chain head instead of invalid access to
  nonexistent struct member.

* Remove several excess '\' escapes.

* Use RTEMS_SELF instead of undefined SELF.

* Use rtems_task instead of void for task functions.

* Add missing stack size parameter in task creation.

* Use rtems_interrupt_level instead of undefined rtems_interrupt.

* Correct return value format for rtems_object_id_get_api()
  rtems_object_id_get_index() (enum and uint16_t) and also fix
  corresponding print formatting.

* Correct return value documentation for rtems_object_id_get_class(),
  rtems_object_id_get_node() and rtems_object_id_get_index()
  int -> uint32_t.

* Use RTEMS_SUCCESSFUL instead of undefined RTEMS_STATUS_SUCCESSFUL and
  fix return value variable name in rate monotonic examples.

* Use RTEMS_TIMEOUT instead of undefined TIMEOUT and RTEMS_PERIOD_STATUS
  instead of undefined STATUS.

* Add missing fields to ftpd configuration.

* Correct parameter types in ftpd hook prototype,
  unsigned char * -> void *.

* Fix various code-block:: attributes, c -> makefile and c -> shell.

* Add missing parenthesis in socket buffer memory calculation example.

* Remove typedef in declaration of mq_attr since it is defiend without
  typedef in reality.

* Update siginfo_t declaration to match current reality.

* Update shell user command definition example to include mode, uid and
  gid.

posix-users/message_passing.rst

@@ -77,7 +77,7 @@ queue.
 
 .. code-block:: c
 
-    typedef struct mq_attr{
+    struct mq_attr{
         long mq_flags;
         long mq_maxmsg;
         long mq_msgsize;

posix-users/signal.rst

@@ -400,16 +400,12 @@ The prototype of the ``siginfo_t`` structure is the following:
     {
         int si_signo;        /* Signal number */
         int si_code;         /* Cause of the signal */
-        pid_t si_pid;        /* Sending process ID */
-        uid_t si_uid;        /* Real user ID of sending process */
-        void* si_addr;       /* Address of faulting instruction */
-        int si_status;       /* Exit value or signal */
         union sigval
         {
             int sival_int;   /* Integer signal value */
             void* sival_ptr; /* Pointer signal value */
         } si_value;          /* Signal value */
-    }
+    } siginfo_t;
 
 **NOTES:**
 
@@ -778,16 +774,12 @@ The prototype of the ``siginfo_t`` structure is the following:
     {
         int si_signo;        /* Signal number */
         int si_code;         /* Cause of the signal */
-        pid_t si_pid;        /* Sending process ID */
-        uid_t si_uid;        /* Real user ID of sending process */
-        void* si_addr;       /* Address of faulting instruction */
-        int si_status;       /* Exit value or signal */
         union sigval
         {
             int sival_int;   /* Integer signal value */
             void* sival_ptr; /* Pointer signal value */
         } si_value;          /* Signal value */
-    }
+    } siginfo_t;
 
 .. _sigtimedwait:
 
@@ -837,7 +829,7 @@ The ``timespec`` structure is defined as follows:
     {
         time_t tv_sec; /* Seconds */
         long tv_nsec;  /* Nanoseconds */
-    }
+    };
 
 **NOTES:**
 
@@ -895,7 +887,7 @@ The ``sigval`` union is specified as:
     {
         int sival_int; /* Integer signal value */
         void* sival_ptr; /* Pointer signal value */
-    }
+    };
 
 **NOTES:**