systemcl: add support for setting BlockIODeviceWeight for unit

This patch allows user to set up BlockIODeviceWeight for unit
through systemctl. Such as

systemctl set-property sshd.service BlockIODeviceWeight="/dev/sda 100"
systemctl: show BlockIODeviceWeight for unit

We can use systemctl show unitname to show the BlockIODeviceWeight
of unit.
man: wording and grammar updates

This includes regularly-submitted corrections to comma setting and
orthographical mishaps that appeared in man/ in recent commits.

In this particular commit:
- the usual comma fixes
- expand contractions (this is prose)