cpukit/shell: Replace task variables with posix keys.

Use posix keys for current shell environment instead of task variables. With
this patch the shell needs one posix-key and one posix-key-value-pair
configured.

Update documentation for the shell.

Adapt samples/fileio:
- Add necessary objects.
- Add login function and custom device name for better testing of the shell.

testsuites/samples/fileio/init.c

@@ -708,10 +708,11 @@ static void fileio_start_shell(void)
     "SHLL",                          /* task_name */
     RTEMS_MINIMUM_STACK_SIZE * 4,    /* task_stacksize */
     100,                             /* task_priority */
-    "/dev/console",                  /* devname */
+    "/dev/foobar",                   /* devname */
+    /* device is currently ignored by the shell if it is not a pty */
     false,                           /* forever */
     true,                            /* wait */
-    NULL                             /* login */
+    rtems_shell_login_check          /* login */
   );
 }
 #endif /* USE_SHELL */

testsuites/samples/fileio/system.h

@@ -47,6 +47,9 @@ rtems_task Init(
 #define CONFIGURE_ATA_DRIVER_TASK_PRIORITY  14
 #endif
 
+#define CONFIGURE_MAXIMUM_POSIX_KEYS             1
+#define CONFIGURE_MAXIMUM_POSIX_KEY_VALUE_PAIRS  1
+
 #if FILEIO_BUILD
   #define CONFIGURE_APPLICATION_NEEDS_LIBBLOCK
   #define CONFIGURE_BDBUF_MAX_READ_AHEAD_BLOCKS  2