Treat oneshot systemd services as "active" when "activating"

[Stebalien]

Oneshot systemd services transition directly from activating to dead/inactive and never show up as "active". This change treats these services as "active" when "activating" because that's likely what the user wants. Otherwise, backup tasks, etc. never show up as active services in i3status-rs.

[Stebalien]

I read this property every time in case the underlying service changes.

[Stebalien]

From the systemd.service man page (do you don't have to go looking for it):

Behavior of oneshot is similar to simple; however, the service
manager will consider the unit up after the main process exits.
It will then start follow-up units. RemainAfterExit= is
particularly useful for this type of service. Type=oneshot is
the implied default if neither Type= nor ExecStart= are
specified. Note that if this option is used without
RemainAfterExit= the service will never enter "active" unit
state, but will directly transition from "activating" to
"deactivating" or "dead", since no process is configured that
shall run continuously. In particular this means that after a
service of this type ran (and which has RemainAfterExit= not
set) it will not show up as started afterwards, but as dead.