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

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@paquier.xyz>