pgsql: Fix EXEC BACKEND + Windows builds for group privs

Fix EXEC BACKEND + Windows builds for group privs

Under EXEC BACKEND we also need to be going through the group privileges
setup since we do support that on Unixy systems, so add that to
SubPostmasterMain().

Under Windows, we need to simply return true from
GetDataDirectoryCreatePerm(), but that wasn't happening due to a missing
 #else clause.

Per buildfarm.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2b74022473f0c7a7fbe11a4973f80478226275bd

Modified Files
--------------
src/backend/postmaster/postmaster.c | 8 ++++++++
src/common/file_perm.c              | 9 +++++++--
2 files changed, 15 insertions(+), 2 deletions(-)