[PATCH] gpg-agent storage - add support for /run based sockets V4

Updated patch. It parses "gpgconf --list-dir" output to find gpg-agent socket.

Now it also unescapes the path according to gpgconf man page which
says the path is percent-escaped.

I'm not sure about using svn_cstring_split on svn_stringbuf_t data
member though, but I could not find a similar functionality for string
buffer.

[[[
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.
]]]