Add support for auto-detetcing OS heading for progress

This patch removes the cognitive overhead of having to manually set your
OS heading, --with-heading="Foo OS vX.YY".  As of this patch, Finit by
default extracts PRETTY_NAME from /etc/os-release.  It is now possible
to also disable the heading entirely using --without-heading

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/helpers.h

@@ -68,6 +68,8 @@ char   *strip_line      (char *line);
 void    enable_progress (int onoff);
 void    show_progress   (pstyle_t style);
 
+char   *release_heading (void);
+
 speed_t stty_parse_speed(char *baud);
 void    stty            (int fd, speed_t speed);