report when zone reload already in progress

report when zone reload already in progress

if a zone reload is already in progress when 'rndc reload <zone>' is
run, currently the message returned in "zone reload queued", which
is correct, but it's identical to the message returned when a reload
was *not* in progress, so the user can't easily tell what happened.
a user could reload a zone twice and not realize that only one
reload actually took place.

this has been addressed by changing the message returned to
"zone reload was already queued".

a new result code ISC_R_LOADING has been added to signal this
condition, taking the place of ISC_R_RELOAD, which was obsolete
and has been removed.