shared/install: when unit contains only Also=, report 'indirect'

If a unit contains only Also=, with no Alias= or WantedBy=, it shouldn't
be reported as static. New 'indirect' status shall be introduced.

https://bugzilla.redhat.com/show_bug.cgi?id=864298
man: improve documentation for "indirect" unit file state a bit

Also, correct mentions of "units" instead of "unit files" in the table,
and terminate all sentences with a full stop.
shared: explicitly ignore the return value of wait_for_terminate

CID#1237532
CID#1237523
CID#1237522