agent,dirmngr: Add "getenv" to the getinfo command.

* agent/command.c (cmd_getinfo): Add sub-command getenv.
* dirmngr/server.c (cmd_getinfo): Ditto.

It is sometimes helpful to be able to inspect certain envvars in a
running agent. For example "http_proxy".

• Signed-off-by: Werner Koch <wk@gnupg.org>