This commit adds the option of having another set of global custom hooks
along with the already supported repository local custom hooks.

The repository local custom hook is executed first (if available). If
successful, execution continues with the global custom hook (if available).
This way, local custom hooks get priority over global custom hooks.

Global custom hooks can be enabled by placing an executable file into the
"custom_hooks" directory within gitlab-shell (create if it does not exist,
yet).

This commit takes the GitlabCustomHook a bit clother to the other hook
handling classes by receiving the repo_path as argument to initialize()
instead of passing it to each method.

Neither other code executing in the hook process nor subprocesses of
the hook process need access to the GL_ID / GL_PROTOCOL variables so
we can just unset them the moment we fetch them.