pgsql: Fix reversed check of return value from sync

Fix reversed check of return value from sync

While at it also update the comments in walmethods.h to make it less
likely for mistakes like this to appear in the future (thanks to Tom for
improvements to the comments).

And finally, in passing change the return type of walmethod.getlasterror
to being const, also per suggestion from Tom.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/b935eb7da37254a18c48acc7b07517c8dfbb2339

Modified Files
--------------
src/bin/pg_basebackup/receivelog.c | 5 ++++-
src/bin/pg_basebackup/walmethods.c | 4 ++--
src/bin/pg_basebackup/walmethods.h | 46 +++++++++++++++++++++++++++++++++++++-
3 files changed, 51 insertions(+), 4 deletions(-)