ssh: tweak default controlPath
ssh: tweak default controlPath

Instead of using the hostname `%h`, which can be changed by the
~/.ssh/config file, use the commandline-given hostname `%n`.

This allows to alias a host with different hostnames, which then point
to different configurations. A common use-case for this is if you have
multiple accounts on github with each access to different private repos:

    Host github.com
      IdentitiesOnly yes
      User git
      IdentityFile ~/.ssh/id_rsa

    Host customer.github.com
      IdentitiesOnly yes
      User git
      IdentityFile ~/.ssh/customer
      HostName github.com

Without this change, if a connection was established with the first
github.com alias, then the user would try to pull a repo from the second
account, ssh would re-use the SSH connection which doesn't have access
to that repository.

rycee/home-manager,rycee/home-manager,rycee/home-manager