Conflicts:
	src/or/config.c

Libevent 2.0 has a "changelist" feature that avoids making redundant
syscalls if we wind up doing a lot of event_add/event_del operations
on the same fd in a row.  Unfortunately, due to a weird design
choice in Linux, it doesn't work right with epoll when multiple fds
refer to the same socket (e.g., one is a dup() of the other).  We
don't dup() anything we give to Libevent, though, so it is safe for
us to explicitly turn this feature on.

Conflicts:
	src/common/memarea.c
	src/or/or.h
	src/or/rendclient.c

This will avoid some signed/unsigned assignment-related bugs.

Conflicts:
	src/config/geoip

It's all too easy in C to convert an unsigned value to a signed one,
which will (on all modern computers) give you a huge signed value.  If
you have a size_t value of size greater than SSIZE_T_MAX, that is way
likelier to be an underflow than it is to be an actual request for
more than 2gb of memory in one go.  (There's nothing in Tor that
should be trying to allocate >2gb chunks.)

Found by boboper.