pgsql: Fix backup canceling

Fix backup canceling

Assert-enabled build crashes but without asserts it works by wrong way:
it may not reset forcing full page write and preventing from starting
exclusive backup with the same name as cancelled.
Patch replaces pair of booleans
nonexclusive_backup_running/exclusive_backup_running to single enum to
correctly describe backup state.

Backpatch to 9.6 where bug was introduced

Reported-by: David Steele
Authors: Michael Paquier, David Steele
Reviewed-by: Anastasia Lubennikova

https://commitfest.postgresql.org/13/1068/

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/8de6278d3b7c810fe5d31486491189d88550a2a6

Modified Files
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src/backend/access/transam/xlog.c | 22 ++++++++++++++++++++++
src/backend/access/transam/xlogfuncs.c | 25 ++++++++++---------------
src/include/access/xlog.h | 21 ++++++++++++++++++++-
3 files changed, 52 insertions(+), 16 deletions(-)