Fixups on variable.c

I've committed a bit more to variable.c to handle integer inputs to GUC
parameters (string and float were already supported). I've included the
cvs log message below.

Further changes aren't precluded of course, but the code now supports
string, integer, and floating point inputs to parameters (for those
parameters which can accept them ;).

- Thomas

Convert GUC parameters back to strings if input as integers.
Change elog(ERROR) messages to say that a variable takes one parameter,
rather than saying that it does not take multiple parameters.