patches for CTFv3

Hi,

I've finished a first draft of some patches which extend the CTF
(compact C type format) definitions, making them a bit less compact but
increasing the limit on the number of permitted C types, as an amd64
GENERIC kernel has been close to the limit for a while now, causing
frustrating dtrace failures.  The new format is CTFv3, compatible at
least in principle with binutils libctf.

The patches add support for v3 to various CTF producers and consumers.
The one which adds v3 definitions is https://reviews.freebsd.org/D34360
and the rest can be viewed under the "stack" tab.  In particular, the
limit on the number of representable types grows from 2^{15} to
2^{31} - 1, hopefully enough to last for a while.

The kernel's CTF section grows somewhat.  The (zlib-compressed) on-disk
size for an amd64 kernel increases from 1.02MB to 1.08MB, and the
uncompressed size increases from 2.50MB to 3.31MB.  In other words, the
impact is hopefully unnoticeable if one's not using dtrace, and
otherwise I believe the increase in memory usage isn't prohibitive.

With the patches, libctf and ctfdump handle both v2 and v3.  ctfconvert
is changed to always emit v3, and ctfmerge can merge v2 and v3
containers, always creating a v3 container.  The kernel works with v2 or
v3, i.e., there should be no problem using dtrace with an updated kernel
and an old version of the CTF toolchain.  If it really becomes useful to
be able to request v2 output from ctfconvert and ctfmerge, it could be
added, but I don't have a reason to implement it yet.

Any feedback on the changes would be appreciated.

Thanks,
-Mark