psql: add \pset true/false

Hello hello,

Since the default t/f output for booleans is not very user friendly,
attached is a patch which enables you to do for example the following:

=# \pset true TRUE
Boolean TRUE display is "TRUE".
=# \pset false FALSE
Boolean FALSE display is "FALSE".
=# select true, false;
bool | bool
------+-------
TRUE | FALSE
(1 row)

(For anyone reviewing: I didn't touch the parts of describe.c which do
this for nullPrint:

myopt.nullPrint = NULL;

since I thought it was dubious in the first place, and I don't think we
output booleans in the describe commands.)

.m