journalcl: simplify handling of stdout being a regular file and epoll()

journalcl: simplify handling of stdout being a regular file and epoll()

Let's not check the fd type beforehand, let's instead gracefully handle
if we get EPERM back from epoll_ctl() because the fd doesn't do epoll.
THis should be safer and more generic.

The epoll_ctl(2) man page clearly documents EPERM is being returned in
this case, hence it's safe to check for exactly that case.

Follow-up for: #28644