fix: usr: Report when a zone reload is already in progress

If a zone reload was already in progress when `rndc reload <zone>` was
run, the message returned was "zone reload queued", which was technically
correct, but it was identical to the message returned when a reload
was not in progress. Consequently, a user could issue two reload commands
without realizing that only one reload had actually taken place. This has
been addressed by changing the message returned to "zone reload was already queued".

Closes #5140

Merge branch '5140-report-reload-in-progress' into 'main'

See merge request isc-projects/bind9!10849