Use strtoul(3) when dealing with user_session.uid

There are cases when we need to convert the users UID from a string into
an unsigned int. Previously we'd been using atoi(3) to do this but this
returns an int, use strtoul instead, while this returns an unsigned
long, the uid should never be more than an unsigned int so this should
be OK.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>