Properly close down the connection to gpg-agent.

This avoids a spurious "Connection reset by peer" log message from gpg-agent.

* subversion/libsvn_subr/gpg_agent.c
  (bye_gpg_agent): New function.
  (find_running_gpg_agent, password_get_gpg_agent, password_set_gpg_agent):
    Replace the close() calls with calls to bye_gpg_agent().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1600331 13f79535-47bb-0310-9956-ffa450edef68