[DESIGN]
o Give IODs their own methods to streamline the code and get rid of all
  the special cases in nsock_core.c. This would also make it easier to
  hook operations (typically: override the default iod_connect() method
  to establish a proxy chain).
o Fix the read API
o Rework the filespace code to avoid unneeded data copy. Scatter/gather
  I/O might be useful there.
o The numerous free-lists (one per gh_list, plus others) of a nsock_pool
  should be shared. Actually, More efficient data structures in nsock
  could also simply make them unneeded.
  !! Do detailed profiling before starting any work on the last two
  points.

[PROXY SUPPORT]
o Proper SSL support
o Authentication
o Handle socks4a
  - This requires to figure out how to trigger proxy code without
    resolving target hostname first. The problem is that the proxy code
    is supposed to be a transparent hook of connect()... Extending the
    exported API will probably be needed :(
  - Async hostname resolution available from within nsock would let us
    try clever tricks... I'm not sure whether nsock should provide it
    or if it should simply provide an API to plug an external system.
o Socks5 support
o Some code is copied from ncat. I should move it to nbase.