pgsql: Allow pg_recvlogical --drop-slot to work without --dbname.

Allow pg_recvlogical --drop-slot to work without --dbname.

When pg_recvlogical was introduced in 9.4, the --dbname option was not
required for --drop-slot. Without it, pg_recvlogical --drop-slot connected
using a replication connection (not tied to a specific database) and
was able to drop both physical and logical replication slots, similar to
pg_receivewal --drop-slot.

However, commit 0c013e08cfb unintentionally changed this behavior in 9.5,
making pg_recvlogical always check whether it's connected to a specific
database and fail if it's not. This change was expected for --create-slot
and --start, which handle logical replication slots and require a database
connection, but it was unnecessary for --drop-slot, which should work with
any replication connection. As a result, --dbname became a required option
for --drop-slot.

This commit removes that restriction, restoring the original behavior and
allowing pg_recvlogical --drop-slot to work without specifying --dbname.

Although this issue originated from an unintended change, it has existed
for a long time without complaints or bug reports, and the documentation
never explicitly stated that --drop-slot should work without --dbname.
Therefore, the change is not treated as a bug fix and is applied only to
master.

Author: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/b15ecf4f-e5af-4fbb-82c2-a425f453e0b2@oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c68100aa4313cf1cef826cf2d50f67164d13ebe4

Modified Files
--------------
src/bin/pg_basebackup/pg_recvlogical.c | 9 ++++++---
src/bin/pg_basebackup/t/030_pg_recvlogical.pl | 8 ++++++++
2 files changed, 14 insertions(+), 3 deletions(-)