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.

David Steele

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