I'll have another look after we're done with release but some quick
comments for now (I haven't tried running it yet but am interested)

| Before starting Icinga DB, the included sample configuration must be
| copied to /etc/icingadb/config.yml and altered accordingly:
|
|   # mkdir /etc/icingadb
|   # install -o _icingadb -g wheel -m 0660 \
|       ${PREFIX}/share/examples/icingadb/config.example.yml \
|       /etc/icingadb/config.yml

should be handled in the port with @sample / @mode / @owner / @group

| Icinga Web 2 Integration
| --------------------

not enough -'s to underline

| To integrate Icinga DB into Icinga Web 2, the icinga-web2-module-icingadb
| package is available. Afterwards, follow its pkg-readme to either either use it

"either either"

There's the same text in net/icinga/web2-module-icingadb/pkg/DESCR as
net/icinga/icingadb/pkg/DESCR. Better to describe what the module in
particular does. Probably starting along the lines of "Icinga Web 2
module for Icinga DB" maybe saying a bit more about what it offers.
I think I'd prefer to follow something closer to upstream's naming
"Icinga DB Web" for the package name/port dir.