Rename runscript to openrc-run

This was requested by Debian, because the minicom software, which is
available on Debian and other distros, has a binary named runscript. We
are keeping a backward compatibility symlink for now, but this allows
Debian or any other distro to safely remove the symlink.

X-Gentoo-Bug: 494220
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=494220