shell: make shell_run run shell forever

This change is in preparation to [PR 10788]. PR 10788 will make the
shell exitable which may lead to unexpected behavior in comparison to
previous usage of the shell.

To prevent this, this PR introduces two "new" functions to the shell's
API: `shell_run_once()` and `shell_run_forever()`.

`shell_run_once()` basically has the same behavior as `shell_run()` in
current master: Start a shell and continue reading lines until EOF is
reached.

`shell_run_forever()` wraps around `shell_run_once()` and restarts the
shell if it exits.

`shell_run()` is re-introduced as a back-porting alias for
`shell_run_forever()`.

As a consequence all current calls to `shell_run()` won't exit even
with [PR 10788] merged (which would add EOT as additional exit
condition for `shell_run_once()`).

[PR 10788]: RIOT-OS#10788

sys/include/shell.h

@@ -64,13 +64,41 @@ typedef struct shell_command_t {
 } shell_command_t;
 
 /**
- * @brief           Start a shell.
+ * @brief           Start a shell and exit once EOF is reached.
  *
  * @param[in]       commands    ptr to array of command structs
  * @param[in]       line_buf    Buffer that will be used for reading a line
  * @param[in]       len         nr of bytes that fit in line_buf
  */
-void shell_run(const shell_command_t *commands, char *line_buf, int len);
+void shell_run_once(const shell_command_t *commands, char *line_buf, int len);
+
+/**
+ * @brief           Start a shell and restart it if it exits
+ *
+ * @param[in]       commands    ptr to array of command structs
+ * @param[in]       line_buf    Buffer that will be used for reading a line
+ * @param[in]       len         nr of bytes that fit in line_buf
+ */
+static inline void shell_run_forever(const shell_command_t *commands,
+                                     char *line_buf, int len)
+{
+    while (1) {
+        shell_run_once(commands, line_buf, len);
+    }
+}
+
+/**
+ * @brief           Back-porting alias for @ref shell_run_forever
+ *
+ * @param[in]       commands    ptr to array of command structs
+ * @param[in]       line_buf    Buffer that will be used for reading a line
+ * @param[in]       len         nr of bytes that fit in line_buf
+ */
+static inline void shell_run(const shell_command_t *commands,
+                             char *line_buf, int len)
+{
+    shell_run_forever(commands, line_buf, len);
+}
 
 #ifdef __cplusplus
 }

sys/shell/shell.c

@@ -290,7 +290,8 @@ static inline void print_prompt(void)
     flush_if_needed();
 }
 
-void shell_run(const shell_command_t *shell_commands, char *line_buf, int len)
+void shell_run_once(const shell_command_t *shell_commands,
+                    char *line_buf, int len)
 {
     print_prompt();