pgsql: Don't allow data_directory to be set in postgresql.auto.conf by

Don't allow data_directory to be set in postgresql.auto.conf by ALTER SYSTEM.

data_directory could be set both in postgresql.conf and postgresql.auto.conf so far.
This could cause some problematic situations like circular definition. To avoid such
situations, this commit forbids a user to set data_directory in postgresql.auto.conf.

Backpatch this to 9.4 where ALTER SYSTEM command was introduced.

Amit Kapila, reviewed by Abhijit Menon-Sen, with minor adjustments by me.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fc5cf383d9ae894e3c9e94701ce5dc468d334433

Modified Files
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doc/src/sgml/ref/alter_system.sgml | 10 ++++++++++
src/backend/utils/misc/guc.c | 21 +++++++++++++++------
src/include/utils/guc.h | 1 +
3 files changed, 26 insertions(+), 6 deletions(-)