Rename the rc binary to openrc

Debian requested this rename due to the "rc" binary conflicting with the
"rc" binary from the plan 9 shell.

We also add a deprecation warning to the binary when it is run as rc to
encourage users to switch to openrc instead.

X-Gentoo-Bug: 493958
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=493958

man/rc_plugin_hook.3

@@ -44,7 +44,7 @@ is set to the name of the runlevel or name of the service.
 Plugins can affect the parent environment by writing NULL separated strings to
 .Va rc_environ_fd .
 .Sh SEE ALSO
-.Xr rc 8 ,
+.Xr openrc 8 ,
 .Xr runscript 8
 .Sh AUTHORS
 .An Roy Marples <roy@marples.name>