pgsql: Make be-secure-common.c more consistent for future SSL implement

Make be-secure-common.c more consistent for future SSL implementations

Recent commit 8a3d9425 has introduced be-secure-common.c, which is aimed
at including backend-side APIs that can be used by any SSL
implementation. The purpose is similar to fe-secure-common.c for the
frontend-side APIs.

However, this has forgotten to include check_ssl_key_file_permissions()
in the move, which causes a double dependency between be-secure.c and
be-secure-openssl.c.

Refactor the code in a more logical way. This also puts into light an
API which is usable by future SSL implementations for permissions on SSL
key files.

Author: Michael Paquier <michael(at)paquier(dot)xyz>

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/2764d5dcfa84d240c901c20ec6e194f72d82b78a

Modified Files
--------------
src/backend/libpq/be-secure-common.c | 74 ++++++++++++++++++++++++++++++++++++
src/backend/libpq/be-secure.c | 69 ---------------------------------
src/include/libpq/libpq.h | 3 +-
3 files changed, 76 insertions(+), 70 deletions(-)