multimedia/jellyfin: adjust pkg-message to include proper jail parameters

In order to run in a jail, jellyfin needs the jail's parameter
allow.mlock to be set, the pkg-message.in mentions this to be
allow_mlock=1, which won't work. Fix this and also use
service jellyfin enable instead of the sysrc line.

PR: 273973
Approved by: michiel@vanbaak.eu