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.