Better title output for psql \dt \di etc. commands

Please find attached a patch to enable more intuitive titles when using
psql and doing the following actions:

\dt \di \dv \dm \ds \dE

In other words, everything in listTables()

We still default to "List or relations", but if we know that the user has
requested exactly one specific type of relation, we use that instead. So
instead of

greg=# \dv
List of relations
Schema | Name | Type | Owner
--------+-------------------------+------+-------
public | pg_stat_statements | view | greg
public | pg_stat_statements_info | view | greg

we get:

greg=# \dv
List of views
Schema | Name | Type | Owner
--------+-------------------------+------+-------
public | pg_stat_statements | view | greg
public | pg_stat_statements_info | view | greg
(2 rows)

The same applies for "misses" as well:

greg=# \di
Did not find any indexes.

greg=# \di foo*
Did not find any indexes named "foo*".

I thought about keeping that last one as singular, as it is replacing "Did
not find any relation named", but not only does it make more sense as a
plural given that wildcards can produce multiple relations, it makes the
code cleaner. :)

Inspired by a recent thread[0] over in pgsql-bugs entitled "Bug in psql".
(while not a bug, it is certainly a welcome enhancement, IMHO).

Cheers,
Greg

[0]
https://www.postgresql.org/message-id/CADrHaBEVSYwemoJWtry2%2B82KHy9tZirH2PVfi-uD96R5J8FCsw%40mail.gmail.com