vcsh: Add config

A small config. Don't generate gitignore files. And use 'hook.d' as hook
directory, not 'hooks-available'.
vcsh: Add post-upgrade hook

- Ignore everything by `echo '*' > $GIT_DIR/info/exclude`.
- Track hooks in $GIT_WORK_TREE/.githooks.d/$NAME.
hooks/post-checkout: Fix permissions of ~/.ssh

chmod -R go= ~/.ssh
vcsh: Move hook.d -> vcsh.hooks.d
vcsh: Move git hooks dir to ~/.config/vcsh/git-hooks.d

I dislike keeping metadata for vcsh repos in "~/.git${label}.d".
Instead, the metadata should be located near all the other vcsh data, in
"~/.config/vcsh".

To remove ambiguity, rename the vcsh hook dir:
  ~/.config/vcsh/hook.d -> ~/.config/vcsh/vcsh-hooks.d