Add vld_ prefix for vldoor_e enum values.

It was pointed out to me recently that vldoor_e has enum values
named 'open' and 'close'. These can potentially conflict with POSIX
standard functions that have the same names, if the right header
files are included.

This doesn't currently cause any problems. However, to avoid any
possibility of a conflict occurring if a different compiler is used,
add a vld_ prefix to all the enum values, to namespace them.