Freeze exception for resolv.conf parser changes

[Florian Weimer <fweimer at redhat dot com>]

I've just posted my resolv.conf changes.  They address two long-standing
problems with the stub resolver: automatic reloading of struct
resolv_conf, and the hard search list limit (6 domains).  Both issues
have seen quite a bit of user interest over the years.

I think these patches are ready to go in now.  The whole construction is
a bit baroque, but that's the price we pay for ABI compatibility and
support for applications which patch _res today.

But I would like to continue working on this during the first week of
the freeze.  I see the following five immediate improvements:

* More test coverage.  (I tested resolv.conf reloading only manually.)
* Lazy allocation of the main state in resolv/resolv_conf.c, to conserve
  data segment space for programs which do not use the stub resolver.
* Eliminate some locking in resolv/resolv_conf.c.
* Use a free list for indexes into the resolv_conf array (speeds
  up _res object allocation/resolv_conf attachment).
* Reimplement the libresolv compat gethostname on top of NSS.

I think these changes are low risk.

There are some additional to-do items, but I don't think they need to
get into the release.

* Clean up the nssocks array (we do not need it).
* Support arbitrarily many name servers.
* Lazily initialize _res on first access.  (Then if *_res is NULL,
  the application obviously has not patched _res.)
* Lazily allocate _res (like global-dynamic TLS).  This frees about ~500
  bytes in the TCB.

Thanks,
Florian