pgsql: Allow restore_command parameter to be changed with reload.

Allow restore_command parameter to be changed with reload.

This commit changes restore_command from PGC_POSTMASTER to PGC_SIGHUP.

As the side effect of this commit, restore_command can be reset to
empty during archive recovery. In this setting, archive recovery
tries to replay only WAL files available in pg_wal directory. This is
the same behavior as when the command that always fails is specified
in restore_command.

Note that restore_command still must be specified (not empty) when
starting archive recovery, even after applying this commit. This is
necessary as the safeguard to prevent users from forgetting to
specify restore_command and starting archive recovery.

Thanks to Peter Eisentraut, Michael Paquier, Andres Freund,
Robert Haas and Anastasia Lubennikova for discussion.

Author: Sergei Kornilov
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/2317771549527294@sas2-985f744271ca.qloud-c.yandex.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/942305a36365433eff3c1937945758f2dbf1662b

Modified Files
--------------
doc/src/sgml/config.sgml                      | 3 ++-
src/backend/utils/misc/guc.c                  | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 1 -
3 files changed, 3 insertions(+), 3 deletions(-)