gpg: Clean up dangling agent_open and agent_closed declarations.

* g10/keydb.h: Remove agent_open, agent_close declarations/
* g10/migrate.c: #include <unistd.h> for access()

--
agent_open() is only defined statically in common/simple-pw-query.c,
it is neither used nor referenced anywhere else.  agent_close doesn't
exist anywhere.  The removal of these declarations removes an
unecessary inclusion of libassuan.h.

migrate.c was relying on keydb.h -> libassuan.h -> unistd.h for the
declaration of access(), so we now handle that explicitly instead.

g10/migrate.c

@@ -22,6 +22,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <unistd.h>
 #include <assert.h>
 
 #include "gpg.h"