Use "runuser" instead of "su" for oracle RA

Similar to issue #633 and #632, I'd like to request to use `runuser` (if available) instead of `su` for the oracle RA as well. We did face similar issues where the PAM configuration of su was causing unnecessary calls to a directory server, which is completely unnecessary for service monitoring using a local account.

A workaround is to place a link from `/usr/lib/ocf/lib/heartbeat/su` to `/sbin/runuser`, but this feels a bit intrusive.

Compare the oracle resource agent with the related approaches in issues #633 and #632, focusing on how it invokes `su`. Check whether `/sbin/runuser` is available and consider the existing workaround at `/usr/lib/ocf/lib/heartbeat/su`; done means monitoring works without unnecessary PAM directory-server calls while retaining a safe fallback when `runuser` is unavailable.

Written by the indexing model from the issue text.