\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.