private-use.dat: generalize the Vietnamese accents

The eSpeak text-to-speech engine defines two Vietnamese dialects:

    vi-hue (Hue province dialect)
    vi-sgn (Saigon province dialect)

to represent Central and Southern Vietnamese accents. These were
mapped by Cainteoir text-to-speech as:

    vi-hue => vi-x-hueprovince
    vi-sgn => vi-x-saigon

The hueprovince private use tag is malformed (longer than 8
characters) and saigon is too specific.

Vietnamese from Vietnam has 5 generalised accents by region:

    vi-VN-x-north
    vi-VN-x-northcnt
    vi-VN-x-central
    vi-VN-x-southcnt
    vi-VN-x-south

Therefore, the eSpeak accents are now mapped to:

    vi-hue => vi-VN-x-central
    vi-sgn => vi-VN-x-south

NOTE: The new private-use values are not specific to Vietnamese.