[GRASS5] pg transaction

[Radim Blazek]

I have added db_begin_transaction() and db_commit_transaction(), currently implemented
in pg driver only and ignored by others. Used in v.to.db and v.what.rast now. 
A bit faster (10000 points, pg driver):
v.to.db option=cat (old)      1m 44.0s
v.to.db option=cat (now)          3.4s
v.what.rast (old,  no index)  4m 57.4s
v.what.rast (old, cat index)  2m 24.4s
v.what.rast (now,  no index)  3m  0.6s
v.what.rast (now, cat index)     17.4s

More modules later.

Radim