Check the current user in SysV init script

While SOLR-14410 is still open I propose a quick fix/improvement for init.d script - check the current user and, if it is the same as RUNAS user, then don't execute "su".

Background:

Systemd has backward compatibility with SysV and able to run scripts from /etc/init.d, but SELinux policies in many distros encourage changing user before this stage and prohibits executing of "su" binary, so it would be logical to do this at systemd level (/etc/systemd/system/solr.service.d/override.conf). In this case, the current init.d script for Solr is missing one very trivial check - `"$RUNAS" != "$USER"`. See the diff-file in the attachment.

Pull request: https://github.com/apache/lucene-solr/pull/1627