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* ssh-agent: add `:omz:plugins:ssh-agent ssh_add_args` option

* Clean up and document extra setting

* Document valid ssh-add arguments
Co-authored-by: Marc Cornellà <hello@mcornella.com>

This change makes the plugin check if an identity is loaded by looking
first at the key filename reported by `ssh-add -l`. This fixes the use
case where ssh-keygen is not able to output the fingerprint of a key,
such as the one reported on #7516.

Now, for an identity to be passed onto ssh-add, it has to fail the
match for a loaded identity, both filename and signature.

With this PR the ssh-agent plugin loads all identities which are not yet
loaded in a single call to ssh-add. If a passphrase is shared between
loaded identities it only needs to be entered once.

Fixes #7506

Use fingerprint of ssh key instead of file name to control if the key is already loaded.

Also check for .ssh folder presence (#5128)

With this PR the ssh-agent plugin checks the `ssh-add -l` output for the
identities added, and adds all those specified by the user that haven't been
added yet.

We also decouple the logic of starting ssh-agent from the logic of adding
identities, meaning that even if ssh-agent has been started by some other means
(like launchd) we can still ssh-add the user's identities.

Fixes #3019
Fixes #6979

* use https everywhere

* use https links on the files that are left

Also, removed some broken links and updated redirections.

When invoking a shell as root using ```sudo -s```, the ssh-agent plugin
starts a new agent although it already exists.

The problem boils down to a check if ssh-agent is running using
```ps x```. If that is extended to ```ps ax``` for root, then the
existing ssh-agent will still work.

The alternative is using tools that aren't available everywhere.
The latest report is that cygwin/msys2 doesn't have pgrep.

Fixes #5418.

Confirmed to work on MacOS, OpenBSD, Solaris and busybox.

On systems where the shell cannot be changed because of a strict
security policy, ssh-agent will use the syntax of whatever the
default $SHELL is.
For instance, if the default shell is tcsh, ssh-agent will use the
c-shell style (setenv).

This change forces ssh-agent to use bourne-style syntax since that
has to be later interpreted by zsh. Consequently, the environment
file will contain `export' statements from now on (instead of
`setenv').

This change is important when ssh-add is not inside /usr/bin e.g.
on NixOS.
Signed-off-by: Maximilian Güntner <code@maschinenpsychologe.de>

On an OS X laptop, the variable `$HOST` changes a lot depending
on what wifi network you're connected to.  This causes a lot
of `~/.ssh/environment-$HOST` files to be created and
causes multiple ssh-agents to created.

Instead, use `scutil --get ComputerName` to get something
more stable.

By default, ssh-agent stores identities forever. It has an option to
set a maximum lifetime for identites (useful to expire passphrase protected
keys). Allow this option to be set using:

zstyle :omz:plugins:ssh-agent lifetime <time>

OpenBSD doesn't have -ef flags for ps. Both linux and OpenBSD have -x flags which works just as greate here

Reorganizing plugins so that each plugin has it's own directory now so that any plugin-specific functions can be bundled within there.