Split proxy.c into smaller files
    
    proxy.c was growing too large to manage (and some graphical
    development tools could no longer open it because of memory
    limitations).
    
    This patch splits proxy.c into the following files:
    proxy_init.c:   Setup routines for the plugin
    proxy_id.c:     Functions to handle user and group lookups
    proxy_auth.c:   Functions to handle PAM interactions
    proxy_common.c: Common utility routines