effective_cache_size patch

I enclose a doc patch for the effective_cache_size parameter in
runtime.sgml: efcdoc.patch

Also, another minor patch which prevents effective_cache_size and
random_page_cost from being set incorrectly: plancost.patch
- previously it was possible to set effective_cache_size to 0, which
would then be ignored and treated as 1 at run-time, so set minimum to 1
and remove test at run-time.
- previously it was possible to set random_page_cost as a fractional
value between 0 and 1, which screws up planner estimation, so set
minimum value of 1 (i.e. random cost same as sequential cost).

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Best Regards, Simon Riggs