Add lua expect(string)

Add simple expect functionality.

The expect(string) function will wait for input from the tty device and
only return when there is a string match. Regular expressions are
supported.

Example:

script = expect('password:'); send('my_password\n')

src/misc.h

@@ -32,3 +32,4 @@ int ctrl_key_code(unsigned char key);
 void alert_connect(void);
 void alert_disconnect(void);
 bool fs_dir_exists(const char *path);
+bool regex_match(const char *string, const char *pattern);