journald: set a lower size limit for FDs from unpriv processes

journald: set a lower size limit for FDs from unpriv processes

Unprivileged processes can send 768M in a FD-based message to journald,
which will be malloc'ed in one go, likely causing memory issues.
Set the limit for unprivileged users to 24M.

Allow coredumps as an exception, since we always allowed storing
up to the 768M max core files in the journal.

Reported on yeswehack.com as #YWH-PGM9780-48

(cherry picked from commit e67b008fa35d92937b07c6b3903419cd42ef9103)