auth external: Resolve current user to cookie

[mpotra]

Currently, the connection to the unix socket fails when using External auth mech for any user that does not have the UID 1000 The reason is that the default cookie is set for UID 1000, and for example root user has UID 0 which fails the auth.

While there is the option to provide a different cookie via the external_cookie application env variable, this PR introduces the system_user value as a way of automatically attempting to determine the correct UID and set the hex-encoded cookie accordingly.

The steps are, somewhat similar to what session_sha1 mech does, but expanded:

• Fetch the OS USER env variable
• Attempt resolving the username to a UID using id -u <user> command
• If successful, hex encode that into a cookie
• If not successful, use the default ?cookie

This way, by simply setting application:set_env(dbus, external_cookie, system_user) from any application that relies on erlang-dbus, the UID is resolved automatically or fails gracefully to the default.

[mpotra]

lgtm, I am wondering why you using custom run_command function, but this is ok for me.

In case run_command is justified, might it be moved to another module for reuse ?

Ah, indeed, there was no other need for it, so I've removed it and placed the body within the calling function. Thanks for pointing it out to me