[Experimental] Scute supporting multiple devices

[NIIBE Yutaka]

Hello,

In GnuPG 2.3, scdaemon supports multiple devices.  Naturally, people
would expect use of multiple devices with Scute.  But it is not (yet)
supported.

This is a ticket for this experiment:

	https://dev.gnupg.org/T6002

Now, we have t6002 branch in the repo of Scute.  Still, there are many
things of FIXME, it starts working (at least, for me).  I tested with
Firefox and Chromium.

For now, you need to install current master of GnuPG 2.3 to use t6002
branch of Scute.

In this experiment, my approach is:

   - It's scdaemon which takes care of device removal.

   - Scute does nothing for device removal/insertion.  All devices are
     marked as "present" even if it's not plugged-in to the host
     computer.

   - When a user asks signing, Scute simply relays the request to
     gpg-agent, and if it's not available, gpg-agent prompts a user for
     device insertion.

Please test, and give us your comments (by mailing list or dev.gnupg.org).
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