man: significantly downgrade the Options section in systemd(1)

This structure of the man page originates from the time when systemd was
installed on top of sysvinit systems, and users had an actual chance to
interact with the systemd binary directly. Nowadays it is almost never called
directly, so let's properly explain this in the overview.

The Options section is moved down below the kernel command line, those options
are only needed in special circumstances. Let's refer the reader to the
description of the kernel command line options, and not duplicate the
descriptions (which makes the text longer than necessary and increases chances
for discrepancies).

Systemd is also prominently used as the user manager, let's mention that in the
Overview.

While at it, use "=" only when an argument is required as we nowadays do.