Create a FETCH_COUNT parameter that causes psql to execute SELECT-like

Create a FETCH_COUNT parameter that causes psql to execute SELECT-like
queries via a cursor, fetching a limited number of rows at a time and
therefore not risking exhausting memory.  A disadvantage of the scheme
is that 'aligned' output mode will align each group of rows independently
leading to odd-looking output, but all the other output formats work
reasonably well.  Chris Mair, with some additional hacking by moi.