pgsql: Use PostmasterRandom(), not random(), for DSM control segment ID

Use PostmasterRandom(), not random(), for DSM control segment ID.

Otherwise, every startup gets the same "random" value, which is
definitely not what was intended.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/626312d1b3ee0c94f7ed685f751878d810ab53fc

Modified Files
--------------
src/backend/postmaster/postmaster.c | 3 +--
src/backend/storage/ipc/dsm.c       | 3 ++-
src/include/postmaster/postmaster.h | 1 +
3 files changed, 4 insertions(+), 3 deletions(-)