dbus: limit the number of env variables to something reasonable, vol. 2

dbus: limit the number of env variables to something reasonable, vol. 2

Turns out we can utilize this limit at a couple more places, so let's
move the previously defined limit constant to env-util.h and use it to
guard a couple more D-Bus methods. Also, bump it a bit, given it's meant
to be a safety cap that can't be hit in valid scenarios.

Follow-up for 7671b43cb88532cce2aa9ad12f777922206d6a42.