pgsql: Disallow setting archive_library and archive_command at the same

Disallow setting archive_library and archive_command at the same time

Setting archive_library and archive_command at the same time is now an
error. Before, archive_library would take precedence over
archive_command.

Author: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/20220914222736.GA3042279%40nathanxps13

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d627ce3b706de16279d8eb683bfeda34ad7197fe

Modified Files
--------------
doc/src/sgml/config.sgml | 10 +++++++---
src/backend/postmaster/pgarch.c | 12 ++++++++++++
2 files changed, 19 insertions(+), 3 deletions(-)