dbus: Hardcode EXTERNAL auth using the uid to avoid dynamic linking

The default mode of dbusConn.Auth(nil) is to use EXTERNAL and
DBUS_COOKIE_SHA1. This code calls user.Current() which gives this
warning when linking docker:

/var/tmp/go-link-mIsVGq/000005.o: In function `mygetpwuid_r':
/usr/local/go/src/pkg/os/user/lookup_unix.go:72: warning: Using 'getpwnam_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
/var/tmp/go-link-mIsVGq/000005.o: In function `_cgo_bc43c179ae62_Cfunc_mygetpwuid_r':
/usr/local/go/src/pkg/os/user/lookup_unix.go:27: warning: Using 'getpwuid_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
Created binary: /go/src/github.com/dotcloud/docker/bundles/0.9.0-dev/binary/docker-0.9.0-dev

For the systemd we use EXTERNAL anyway, which only needs the uid, not
the username. So we set our own list of methods to avoid running
this code.
Merge pull request #39 from alexlarsson/avoid-dynamic-linking

dbus: Hardcode EXTERNAL auth using the uid to avoid dynamic linking