Fixing a redundancy when using a Keyspan USB-serial adapter

Keyserial1 is an alias to the first port on your Keyspan adapter
(regardless if it's single- or multi-port). USA19* is a prefix to the
raw port on a single-port adapter. Therefore it's redundant to have
both in PortDB. In fact it's an error when FindSerialPort finds > 1
match by default.
Merge pull request #19 from zacklb/findserial-fix

Fixing a redundancy when using a Keyspan USB-serial adapter