pgsql: Use an enum instead of two bools to indicate wal inclusion in ba

Use an enum instead of two bools to indicate wal inclusion in base backups

This makes the code easier to read as it becomes more explicit what the
different allowed combinations really are.

Suggested by Michael Paquier

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/534b6f3ef29a11d19deeef6cef3bc8d4984de3c8

Modified Files
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src/bin/pg_basebackup/pg_basebackup.c | 38 ++++++++++++++++++++---------------
1 file changed, 22 insertions(+), 16 deletions(-)