Add configurable PAM service name

Currently, ocserv uses a hardcoded PAM service name when the pam auth method is selected. In virtual hosts deployments, it is often necessary to have different PAM configurations for different virtual hosts (e.g., using different LDAP groups or enforcing 2FA for some vhosts).

I propose adding a service sub-option to the auth = "pam[...]" directive. This allows each virtual host to point to a specific PAM configuration file, enabling per-vhost PAM stacks.

Example:

auth = "pam[service=vpn1,gid-min=1000]"