o5m output: write reset flag each time a different type (node,way,relation)
is written. This might have a negative impact on output file size for 
"mixed" input files, but seems to be needed.

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@313 838fabe5-b1a8-4b81-9896-7a84386980e9