gpg-agent: Show PID/command line of the requesting process

[Yuri D'Elia]

When using gpg-agent in daemon mode it's not always obvious which
process is requesting authorization for unlocking a key. The agent
should always show the PID/command of the requester.

I filed a bug report at Debian:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=773716

and I also decided to give it a try in implementing the fix.
In the attached patch I have the first crude proposal:

- A new struct is introduced in the main context, adding the PID and the
command line of the requesting process.
- As soon as the connection is established, the fields are populated.

Of note here, is that libassuan does have PID/gid/uid information
already (Using various forms of SO_PEERCRED/getpeercred/etc), but it's
established only later in the individual handlers once an assuan context
is created. Since I'm aiming to make at least the PID *mandatory*, I
think it's useless to perform this task for each handler. It's also true
that copying the entire #ifdef block from libassuan is bad as well...

The command line is extracted from /proc/[pid]/cmdline. Does somebody
know if BSDs have something equivalent? If not, inserting the path of
the executable associated with the PID would be the way to go.

- When constructing the description for the ssh daemon, the PID/process
is also shown in the text.
- The modifiers %p/%P are introduced to insert the PID and command line
into the prompt.

When unlocking a secret key through the normal handler though, the
prompt is generated by the calling process.

It would make sense to me, similarly as done for SSH, that a minimum
description (pid/program/key/operation) must be inserted by the agent
itself, and not by the calling process (which could be easily faked).

Having a fixed text would be easy (such as: Program [PID] [command] is
requesting to unlock the secret key ...). While doing some tests, it
seems to me that I don't lose anything by ignoring completely gpg's prompts.

Comments?
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