pgsql: Allow dbname to be written as part of connstring via pg_baseback

Allow dbname to be written as part of connstring via pg_basebackup's -R option.

Commit cca97ce6a665 allowed dbname in pg_basebackup connstring and in this
commit we allow it to be written in postgresql.auto.conf when -R option is
used. The database name in the connection string will be used by the
logical replication slot synchronization on standby.

The dbname will be recorded only if specified explicitly in the connection
string or environment variable.

Masahiko Sawada hasn't reviewed the code in detail but endorsed the idea.

Author: Vignesh C, Kuroda Hayato
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAB8KJ=hdKdg+UeXhReeHpHA6N6v3e0qFF+ZsPFHk9_ThWKf=2A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a145f424d5248a09d766e8cb503b999290cb3b31

Modified Files
--------------
doc/src/sgml/ref/pg_basebackup.sgml          | 10 +++-
src/bin/pg_basebackup/pg_basebackup.c        | 12 ++++-
src/bin/pg_basebackup/streamutil.c           | 70 ++++++++++++++++++++++++++++
src/bin/pg_basebackup/streamutil.h           |  2 +
src/bin/pg_basebackup/t/010_pg_basebackup.pl | 13 ++++++
src/bin/pg_rewind/pg_rewind.c                |  4 +-
src/fe_utils/recovery_gen.c                  | 21 ++++++++-
src/include/fe_utils/recovery_gen.h          |  3 +-
8 files changed, 127 insertions(+), 8 deletions(-)