psql command aliases support

Folks,

please find attached a patch which implements psql command aliases. They
work the same way as on bash, zsh and others, for example:

#= \alias d \dt+
#= \d
List of relations
Schema | Name | Type | Owner | Description
--------+------+-------+-------+-------------
public | foo | table | bernd |
(1 row)

=# \alias current_query SELECT current_query, NOW() - query_start FROM
pg_stat_activity WHERE current_query NOT LIKE 'IDLE%';

#= \current_query
current_query
| ?column?
-------------------------------------------------------------------------------------------------------+----------
SELECT current_query, NOW() - query_start FROM pg_stat_activity WHERE
current_query NOT LIKE 'IDLE%'; | 00:00:00
(1 row)

#= \unalias d
#= \d
List of relations
Schema | Name | Type | Owner
--------+------+-------+-------
public | foo | table | bernd
(1 row)

I hope i broke nothing and maybe we find this useful for 8.4, documentation
included.

--
Thanks

Bernd