[PATCH] gpg-agent storage - add support for /run based sockets V2

New version. Instead of reimplementing the gpg-agent logic, "gpgconf
--list-dir agent-socket" is used. If the gpgconf fails, the code falls
back to the original solution to stay compatible with old Gnupg
versions.

[[[
Find gpg-agent socket using gpgconf if possible.
This allows detection of socket with Gnupg >= 2.1.13
which changed the default socket path to /run/user/UID/gnupg

* subversion/libsvn_subr/gpg_agent.c
    (find_gpgconf_agent_socket): new function to find
    gpg-agent socket using gpgconf
    (find_running_gpg_agent): use find_gpgconf_agent_socket
    to detect socket when possible.
]]]