agent,common: move get_socket_name() into common.
* agent/gpg-agent.c (get_socket_name): move to ... * common/sysutils.c (gnupg_get_socket_name): ... here.
This allows us to use the same functionality in dirmngr as well.
- Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>