networkd: Add bridge port priority setting (#5545)

networkd: Add bridge port priority setting (#5545)

Allow setting bridge port priority in the Bridge section of the network file,
similar to e.g. port path cost setting.

Set the default to an invalid value of 128, and only set the port priority when
it's not 128. Unlike e.g. path cost, zero is a valid priority value.

Add a networkd-test.py to check that bridge port priority is correctly set.

Incidently, fix bridge port cost type and document valid ranges.