We used strtol() on a number of places to convert into uid_t or gid_t
from a string representation such as LDAP attribute, but on some
platforms, unsigned long might be necessary to store big id_t values.

This patch converts to using strtoul() instead.

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

We were not passing the old authtok to the pam_chauthtok()
function, causing it to return PAM_AUTH_ERR.

We weren't zeroing out the proxy_auth_ctx when we created it, so
the 'running' element was sometimes being filled with garbage data
that exceeded the maximum number of child processes. This meant
that no requests were ever sent to the child processes.

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).