Implements the feature requested in gitlab-org/gitlab#19672

This requires the internal api counterpart in gitlab-org/gitlab!36302 to
be merged first.

It can be used as follows:
```
censored@censored-VirtualBox:~/git/gitlab$ ssh git@gitlab-2004 personal_access_token
remote:
remote: ========================================================================
remote:
remote: Usage: personal_access_token <name> <scope1[,scope2,...]> [ttl_days]
remote:
remote: ========================================================================
remote:

censored@censored-VirtualBox:~/git/gitlab$ ssh git@gitlab-2004 personal_access_token newtoken read_api,read_repository 30
Token:   aAY1G3YPeemECgUvxuXY
Scopes:  read_api,read_repository
Expires: 2020-08-07
```

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

We had `gitlab-shell-authorized-keys-check` and
`gitlab-shell-authorized-principals-check` as symlinks to
`gitlab-shell` before.

We determine the `Command` and `CommandArgs` that we build based
on the `Name` of the `Executable`. We also use that to know which
fallback ruby executable should we fallback to. We use
`os.Executable()` to do that.

`os.Executable()` behaves differently depending on OS. It may
return the symlink or the target's name. That can result to a
buggy behavior.

The fix is to create binaries for each instead of using a symlink.
That way we don't need to rely on `os.Executable()` to get the name.
We pass the `Name` of the executable instead.

Add a new configuration option, custom_hooks_dir. When this is set, we
will look for global custom hooks in:
    <custom_hooks_dir>/{pre-receive,update,post-receive}.d/*

When this is not set, default to <REPO_PATH>/hooks.

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).

`make update-redis` will clone the library and adjust the paths properly

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Something like gitlab-shell.log-20140626.gz will not be caught by the
original pattern match since '.' is taken as a literal character.

This prevents concurrent modification of authorized_keys file

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Ignore gzipped logs as well.