Do not try to continue reading shell commands if input source is close

In RIOT native, sending CTRL+D to a shell started using shell_run would resulted in and
endless prompt loop. I've been unable to trigger such a behaviour
on actual hardware using a UART connection, but calling `pm_off` seemed
like a better alternative than having an `#ifdef BOARD_NATIVE`.

Fixes #9946

sys/shell/shell.c

@@ -31,6 +31,7 @@
 #include <stdlib.h>
 #include "shell.h"
 #include "shell_commands.h"
+#include "periph/pm.h"
 
 #if !defined(SHELL_NO_ECHO) || !defined(SHELL_NO_PROMPT)
 #ifdef MODULE_NEWLIB
@@ -228,7 +229,7 @@ static int readline(char *buf, size_t size)
 
         int c = getchar();
         if (c < 0) {
-            return 1;
+            return EOF;
         }
 
         /* We allow Unix linebreaks (\n), DOS linebreaks (\r\n), and Mac linebreaks (\r). */
@@ -287,6 +288,10 @@ void shell_run(const shell_command_t *shell_commands, char *line_buf, int len)
     while (1) {
         int res = readline(line_buf, len);
 
+        if (res == EOF) {
+            pm_off();
+        }
+
         if (!res) {
             handle_input_line(shell_commands, line_buf);
         }