man,journalctl: introduce man/--help sections

man,journalctl: introduce man/--help sections

So far the --help text and the man page of journactl were mostly a large
pile of options shown next to each other. Let's add some basic
structure, and group switches by sections such as "Filtering Options",
"Output Options" and so on.

Do this the same way in the --help text and in the man page.

Since this moves everything around anyway, I also opted to rebreak all
paragraphs in the man page. This makes the patch larger than necessary,
but given that this whole patch doesn't really change contents besides
section titles I figured this would be OK.