pgsql: Revert exporting of internal GUC variable "data_directory".

Revert exporting of internal GUC variable "data_directory".

This undoes a poorly-thought-out choice in commit 970a18687f9b3058, namely
to export guc.c's internal variable data_directory.  The authoritative
variable so far as C code is concerned is DataDir; there is no reason for
anything except specific bits of GUC code to look at the GUC variable.

After yesterday's commits fixing the fsync-on-restart patch, the only
remaining misuse of data_directory was in AlterSystemSetConfigFile(),
which would be much better off just using a relative path anyhow: it's
less code and it doesn't break if the DBA moves the data directory of a
running system, which is a case we've taken some pains over in the past.

This is mostly cosmetic, so no need for a back-patch (and I'd be hesitant
to remove a global variable in stable branches anyway).

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/da33a3894e0fc440ac53cdc0f2e360e703b13b8c

Modified Files
--------------
src/backend/utils/misc/guc.c |   10 +++++-----
src/include/utils/guc.h      |    1 -
2 files changed, 5 insertions(+), 6 deletions(-)