\prompt for psql

This adds the ability to "prompt" for internal variable input, below are examples:

test=# \prompt x
Enter value for "x": 3
test=# select * from foo where x = :x;
 x
---
 3
(1 row)

test=# \prompt x Enter x:
Enter x: 4
test=# select * from foo where x = :x;
 x
---
 4
(1 row)



====

The input is limited to 4k, should be reasonable enough.  Patch is against CVS HEAD.