bridge: be consistent about PVID terminology

The PVID option is exposed to the user as 'untagged', but the API was
inconsistent on whether it's called 'untagged' or 'PVID'. Standardise
on calling it PVID everywhere in the code, since this is the 802.1Q
terminology.

Keep 'untagged' as the user-facing term since sysadmins are not network
admins and are often not familiar with the term PVID.

Approved by: des
Differential Revision: https://reviews.freebsd.org/D51182