Allow tty's to always respawn, no max restart count

This patch adds a `respawn` flag for services and ttys, always set for
ttys, that allows bypassing the crash/restart counter and immediately
restart a 'crashing' service.

For tty type services this is the expected behavior, but for regular
services it is not.  That is why `respawn` flags is not advertised in
the docs.

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

src/svc.h

@@ -127,6 +127,7 @@ typedef struct svc {
 
 	/* Counters */
 	char           once;	       /* run/task, (at least) once per runlevel */
+	char           respawn;	       /* ttys, or services with `respawn`, never increment restart_cnt */
 	const char     restart_cnt;    /* Incremented for each restart by service monitor. */
 
 	union {