[GRASS5] vector maps: use .tmp directory, then final directory

[Markus Neteler]

Hi,

can we change the vector library to first write into
the .tmp directory (as done for raster data) and then
copy into the target directory within vector/?
Like this we also had the "transactional" file management.

Currently, if a vector command fails (crash, connection to
remote machine fails, disk full or other problems), it leaves
corrupted files which appear to be a valid vector map.

Markus