add argument: default_ssh_auth_sock and some fixes

[edward-p]

Useful when SSH_AUTH_SOCK can't be set in environment. For example: loging in from a Display Manager (such as SDDM).

[kgabor545]

Thank you it solved my problem. I can use pam_ssh_auth_sock in polkit-1.
I just added a little modification to pam_ssh_agent_auth.c:

#include "misc.h"
...
    if(default_ssh_auth_sock && user) {
       default_ssh_auth_sock = pamsshagentauth_percent_expand(default_ssh_auth_sock,
       "h", getpwnam(user)->pw_dir,
       "u", user, NULL);
    }

I'm not sure this is the way to achieve %h, %u expansion in code, but it does it's job.
I make a symlink from the actual socket to the users home with a static name .ssh_auth_sock maybe it could be also a default name if no environment variable.

[pallaswept]

I can use pam_ssh_auth_sock in polkit-1.

I found this very useful, also. pam_ssh_agent_auth won't work with polkit without this patch, because polkit clears the environment.
With this patch, it works with pkexec, and also gksu and kdesu, so it works for all my GUI apps as well as sudo and su in CLI.
The expansion of %U is also very helpful to point pam at the default location of the socket, eg auth sufficient pam_ssh_agent_auth.so file=/root/.ssh/authorized_keys_pam default_ssh_auth_sock=/run/user/%U/ssh-agent.socket

Today, it was brought to my attention that run0 would be in my distro's future, so I checked it out, and it uses polkit, so it looks like this patch is required for pam_ssh_agent_auth to work with systemd-run0. I can confirm that it does work.

Thanks @edward-p