journal: bump RLIMIT_NOFILE when journal files to 16K (if possible)

When there are a lot of split out journal files, we might run out of fds
quicker then we want. Hence: bump RLIMIT_NOFILE to 16K if possible.

Do these even for journalctl. On Fedora the soft RLIMIT_NOFILE is at 1K,
the hard at 4K by default for normal user processes, this code hence
bumps this up for users to 4K.

https://bugzilla.redhat.com/show_bug.cgi?id=1179980