pgsql: Change recovery_init_sync_method to PGC_SIGHUP.

Change recovery_init_sync_method to PGC_SIGHUP.

The setting has no effect except during startup. It's still nice to be
able to change it dynamically, which is expected to be pretty useful to
an admin following crash recovery when restarting the cluster is not so
appealing.

Per discussions following commits 2941138e6 and 61752afb2.

Author: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://postgr.es/m/20210529192321.GM2082%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/34a8b64b4e5f0cd818e5cc7f98846de57938ea57

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(-)