pgsql: Don't use bgw_main even to specify in-core bgworkerentrypoints.

Don't use bgw_main even to specify in-core bgworker entrypoints.

On EXEC_BACKEND builds, this can fail if ASLR is in use.

Backpatch to 9.5.  On master, completely remove the bgw_main field
completely, since there is no situation in which it is safe for an
EXEC_BACKEND build.  On 9.6 and 9.5, leave the field intact to avoid
breaking things for third-party code that doesn't care about working
under EXEC_BACKEND.  Prior to 9.5, there are no in-core bgworker
entrypoints.

Petr Jelinek, reviewed by me.

Discussion: http://postgr.es/m/09d8ad33-4287-a09b-a77f-77f8761adb5e@2ndquadrant.com

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/2113ac4cbb12b815804e8873d761cade9ddf49b9

Modified Files
--------------
doc/src/sgml/bgworker.sgml                 | 36 ++++---------
src/backend/access/transam/parallel.c      |  6 +--
src/backend/postmaster/bgworker.c          | 81 +++++++++++++++++++-----------
src/backend/replication/logical/launcher.c |  6 ++-
src/include/access/parallel.h              |  2 +
src/include/postmaster/bgworker.h          |  5 +-
src/test/modules/test_shm_mq/setup.c       |  1 -
src/test/modules/worker_spi/worker_spi.c   |  4 +-
8 files changed, 75 insertions(+), 66 deletions(-)