Use for name of unnamed portal's memory context

I propose the attached little patch to set the name of the memory 
context of an unnamed portal to "<unnamed>" instead of just an empty 
string.  Otherwise the debug memory context dump looks a little 
confusing.  We use this convention elsewhere for making unnamed 
statements etc. more clearly visible.

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Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services