Fix filesystems not unmounted at shutdown

Instead of unconditionally waiting 2 seconds for processes to die,
check continuously for remaining processes, and break the loop when
none remain.

Turn PID 1 to a RT process with highest priority 99 during shutdown,
this ensures it would not be preempted by other RT processes.

Signed-off-by: Robert Andersson <robert.m.andersson@atlascopco.com>
Signed-off-by: Mathias Thore <mathias.thore@atlascopco.com>
Signed-off-by: Ming Liu <liu.ming50@gmail.com>

src/util.h

@@ -64,6 +64,7 @@ char *sig2str      (int sig);
 char *code2str     (int code);
 
 void  do_sleep     (unsigned int sec);
+void  do_usleep    (unsigned int usec);
 long  jiffies      (void);
 char *uptime       (long secs, char *buf, size_t len);
 char *memsz        (uint64_t sz, char *buf, size_t len);