pgsql: Adjust server-side backup to depend on pg_write_server_files.

Adjust server-side backup to depend on pg_write_server_files.

I had made it depend on superuser, but that seems clearly inferior.
Also document the permissions requirement in the straming replication
protocol section of the documentation, rather than only in the
section having to do with pg_basebackup.

Idea and patch from Dagfinn Ilmari Mannsåker.

Discussion: http://postgr.es/m/87bkzw160u.fsf@wibble.ilmari.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7f6772317b4a4f9d30c25d1853571fca6d834f0c

Modified Files
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doc/src/sgml/protocol.sgml                  | 5 +++++
doc/src/sgml/ref/pg_basebackup.sgml         | 3 ++-
src/backend/replication/basebackup_server.c | 6 ++++--
3 files changed, 11 insertions(+), 3 deletions(-)