This error is typically due to trying to start more that one daemon process or a previous process not exiting cleanly. Thus make absolutely sure there is no other monerod process running, such as running killall monerod, then trying to start again should fix the issue (or a reboot).
What is the bare minimum to mine?
Pretty much any computing device can mine.
What are pools used for
To pool the resources of multiple miners to increase the chance of finding blocks.
and what Linux x86 program could I use to mine Monero?
XMRig for pool or solo mining, the official wallet/daemon for solo.
In case this helps anyone, I had to change the limits by adding a line to /etc/security/limits.conf. For <domain> I put my username for the linux user that is using the wallet, for <type> I put hard, for <item> I put memlock and for <value> I put 2048. I think a minimum of 1024 was needed in my case.
Reboot and now it becomes possible ...