pgsql: Split pg_start_backup() and pg_stop_backup() into two pieces

Split pg_start_backup() and pg_stop_backup() into two pieces

Move the actual functionality into a separate function that's
easier to call internally, and change the SQL-callable function
to be a wrapper calling this.

Also create a pg_abort_backup() function, only callable internally,
that does only the most vital parts of pg_stop_backup(), making it
safe(r) to call from error handlers.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=4448917d51d6f87655ce357cca009634b0a136e4

Modified Files
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src/backend/access/transam/xlog.c | 66 ++++++++++++++++++++++++++++++++----
src/include/access/xlog.h | 4 ++
2 files changed, 62 insertions(+), 8 deletions(-)