Change $prog back to docker in sysvinit-redhat

(and set $exec to dockerd instead)

This ensures end users do not need to make any configuration changes
due to the rename from docker to dockerd in version 1.12.

Signed-off-by: Paul Furtado <pfurtado@hubspot.com>

contrib/init/sysvinit-redhat/docker

@@ -22,9 +22,9 @@
 # Source function library.
 . /etc/rc.d/init.d/functions
 
-prog="dockerd"
+prog="docker"
 unshare=/usr/bin/unshare
-exec="/usr/bin/$prog"
+exec="/usr/bin/dockerd"
 pidfile="/var/run/$prog.pid"
 lockfile="/var/lock/subsys/$prog"
 logfile="/var/log/$prog"