Change PQconndefaults() to return a malloc'd array, instead of a static

Change PQconndefaults() to return a malloc'd array, instead of a static
array.  This allows processing of conninfo strings to be made thread-safe,
at the cost of a small memory leak in applications that use
PQconndefaults() and are not updated to free the returned array via
the new PQconninfoFree() function.  But PQconndefaults() is probably not
used very much, so this seems like a good compromise.