Reverse out XLogDir/-X write-ahead log handling, per discussion.

src/backend/access/transam/xlog.c

src/backend/bootstrap/bootstrap.c

src/backend/postmaster/postmaster.c

src/backend/tcop/postgres.c

src/bin/initdb/initdb.sh

src/bin/pg_ctl/pg_ctl.sh

src/bin/pg_resetxlog/pg_resetxlog.c

src/include/access/xlog.h