Change imports: github.com/guelfey/go.dbus -> github.com/godbus/dbus
auth: Send uid rather than username when authenticating

There is no particular advantage to sending the username. It just
means the other end has to map it to a uid when comparing with the
uid from getpeercred. Additionally that mapping is not always
possible, as the remote side may not have the correct /etc/passwd.
For instance, this can happen when talking to a dbus service inside
a container.
Add static_build tag which avoids user.Current()

Its not safe to use os/user.Current() in a statically linked program.
You will be getting warnings like:

 warning: Using 'getpwnam_r' in statically linked applications requires
  at runtime the shared libraries from the glibc version used for linking
 warning: Using 'getpwuid_r' in statically linked applications requires
  at runtime the shared libraries from the glibc version used for linking

We avoid this when building with -tag static_build by parsing
/etc/passwd directly for the homedir.