Install to /usr/bin

* Standardize all install methods to use /usr/bin rather than some of
  them using /usr/local/bin

* Make the init script executable in install-openrc

systemd/pgmon@.service

@@ -7,7 +7,7 @@ After=network.target
 [Service]
 EnvironmentFile=/etc/pgmon/%i-service.conf
 User=${SERVICE_USER:-postgres}
-ExecStart=/usr/local/bin/pgmon -c /etc/pgmon/%i.yml
+ExecStart=/usr/bin/pgmon -c /etc/pgmon/%i.yml
 ExecReload=kill -HUP $MAINPID
 Restart=on-failure
 Type=exec