pgsql: Allow pg_basebackup from standby node with safety checking.

Allow pg_basebackup from standby node with safety checking.
Base backup follows recommended procedure, plus goes to great
lengths to ensure that partial page writes are avoided.

Jun Ishizuka and Fujii Masao, with minor modifications

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8366c7803ec3d0591cf2d1226fea1fee947d56c3

Modified Files
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doc/src/sgml/ref/pg_basebackup.sgml | 42 ++++
src/backend/access/transam/xlog.c | 404 ++++++++++++++++++++++++++++---
src/backend/postmaster/checkpointer.c | 42 +++-
src/backend/postmaster/postmaster.c | 4 +-
src/backend/replication/basebackup.c | 25 ++-
src/backend/utils/misc/guc.c | 1 -
src/bin/pg_controldata/pg_controldata.c | 5 +
src/bin/pg_resetxlog/pg_resetxlog.c | 7 +-
src/include/access/xlog.h | 2 +
src/include/access/xlog_internal.h | 2 +-
src/include/catalog/pg_control.h | 11 +-
11 files changed, 501 insertions(+), 44 deletions(-)