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

https://fedorahosted.org/sssd/ticket/525

We can't do much about an error here, but we should be reporting
it.

https://fedorahosted.org/sssd/ticket/534

Fixes: #498

This patch adds a new tevent_req to the proxy provider, which will
spawn short-lived child processes to handle PAM requests. These
processes then call the proxied PAM stack and return the results
via SBUS method reply. Once it is returned, the parent process
kills the child.

There is a maximum of ten child processes running simultaneously,
after which requests will be queued for sending once a child slot
frees up. The maximum processes will be made configurable at a
later date (as this would violate string freeze).