Add a gdk doctor check for GDK path length as gitaly can break

We've seen issues recently regarding gitaly not being able to create a socket:

2020-02-13_17:13:21.82959 praefect-gitaly-0     : time="2020-02-13T09:13:21-08:00" level=info msg="Starting Gitaly" version="Gitaly, version 1.86.0"
2020-02-13_17:13:21.83092 praefect-gitaly-0     : time="2020-02-13T09:13:21-08:00" level=fatal msg="load config" config_path=/Users/xxxxxx/src/gitlab.com/gitlab-org/gitlab/gitlab-development-kit/gitaly/gitaly-0.praefect.toml error="socket could not be created in /Users/xxxxxx/src/gitlab.com/gitlab-org/gitlab/gitlab-development-kit/tmp/praefect/gitaly-0: listen unix /Users/xxxxxx/src/gitlab.com/gitlab-org/gitlab/gitlab-development-kit/tmp/praefect/gitaly-0/test-dd5bb029.sock: bind: invalid argument"

With the root cause being the overall path length was too long. We should add a gdk doctor checkup to inspect the length of the GDK path and warn if it's longer than a known bad length.

Related: #1246 (closed)

cc @dcroft, @dsearles, @johncai