Ask GnuPG for the correct path for the uiserver socket

Calling GpgME::dirInfo() which uses 'gpgconf --list-dirs' implicitly
creates the socket directory (since gpg 2.1.20), so we omit a check.
For older versions of gpg (which are unsupported since a long time)
we keep using the old code which may create a gnupg home directory
with unsafe permissions.

GnuPG-bug-id: 5619
BUG: 441957
FIXED-IN: 21.08.2
(cherry picked from commit a0883f36)