databases/mongodb80: harden the port to external CONFIGURE_ARGS

CONFIGURE_ARGS is directly connected to CONFIGURE_SCRIPT and the script
only accepts 2 parameters. Exit 1 on incorrect number of parameters.
And don't allow to += on CONFIGURE_ARGS.

Not bumping PORTREVISION, no content change intended.

NB: This is the same change as 380338c53b75ef594b1faccc59bc111546809f31
for mongodb70.

PR: 291951