Change LimitNOFILE default from 16364 to 16384

This is only a very minor issue, but the default for LimitNOFILE in the SystemD mariadb.service and mariadb@.service files we distribute is 16364, not the expected 16384 (2^16) power-of-two value.

I found it confusing why open_files_limit would be adjusted to 16364 when setting it to a higher value, trying to understand where and why it would lower the expected number by 20 for internal reasons, while it actually didn't change the value it got from SystemD at all ...