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

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
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/0ef26bb394abedb2745bd838c26ecb3131682bda

Modified Files
--------------
src/backend/access/transam/parallel.c | 7 +--
src/backend/postmaster/bgworker.c | 76 ++++++++++++++++++++++++++------
src/include/access/parallel.h | 2 +
src/test/modules/worker_spi/worker_spi.c | 6 ++-
4 files changed, 73 insertions(+), 18 deletions(-)