pgsql: Allow root-owned SSL private keys in libpq, not only the backend

Allow root-owned SSL private keys in libpq, not only the backend.

This change makes libpq apply the same private-key-file ownership
and permissions checks that we have used in the backend since commit
9a83564c5.  Namely, that the private key can be owned by either the
current user or root (with different file permissions allowed in the
two cases).  This allows system-wide management of key files, which
is just as sensible on the client side as the server, particularly
when the client is itself some application daemon.

Sync the comments about this between libpq and the backend, too.

Back-patch of a59c79564 and 50f03473e into all supported branches.

David Steele

Discussion: https://postgr.es/m/f4b7bc55-97ac-9e69-7398-335e212f7743@pgmasters.net

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5bb3d91ea9269dfe8a7f429fe68e1148384f1cbf

Modified Files
--------------
doc/src/sgml/libpq.sgml                  | 26 +++++++++++++++------
src/backend/libpq/be-secure-common.c     | 28 +++++++++++-----------
src/interfaces/libpq/fe-secure-openssl.c | 40 +++++++++++++++++++++++++++++---
3 files changed, 69 insertions(+), 25 deletions(-)