pgsql: Allow the wal_buffers setting to be auto-tuned to a reasonable v

Allow the wal_buffers setting to be auto-tuned to a reasonable value.

If wal_buffers is initially set to -1 (which is now the default), it's
replaced by 1/32nd of shared_buffers, with a minimum of 8 (the old default)
and a maximum of the XLOG segment size. The allowed range for manual
settings is still from 4 up to whatever will fit in shared memory.

Greg Smith, with implementation correction by me.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=0f73aae13def660371c34c8feda6e684e6366bdb

Modified Files
--------------
doc/src/sgml/config.sgml | 25 +++++++++++----
src/backend/access/transam/xlog.c | 41 ++++++++++++++++++++++++-
src/backend/utils/misc/guc.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
4 files changed, 61 insertions(+), 9 deletions(-)