Do not include <stdint.h> nor <inttypes.h> in event2/util.h.

Instead, autoconf determines what fundamental type each fixed-size
typedef is defined as, and then event2/util.h sets up our (existing)
parallel system of typedefs accordingly.  It is guaranteed that on
systems where at least one of <stdint.h> and <inttypes.h> exists, the
ev_* typedefs are compatible (in the C-standardese sense) with the
system typedefs.

ev_intptr_t, ev_uintptr_t, ev_ssize_t, and ev_off_t receive the same
treatment, but we are still including <sys/types.h> in event2/util.h
for the time being.  The end goal for this branch is to include *no*
system headers from non-compat event2/ headers, and to prune
event2/util.h down to things we actually need for the other headers,
plus utility routines that are part of the official API.

There is a new event2/util_compat.h which will preserve everything
that's removed from event2/util.h during this process.
No need to use EV_SIZE_MAX here.
Move EV_SIZE_MAX and evutil_offsetof to util_compat.h.

All code in the library itself switched to use bare offsetof
(EV_SIZE_MAX was already zapped).

Slight change in plan from previous commit log: it doesn't really gain us
anything to try to avoid including stddef.h or stdarg.h in event2/util.h,
and having them there means we don't need to change dozens of declarations
that use bare size_t, or autoconf probe for the underlying definition
of va_list.  So those will remain.