Changeset f21bc36 for variable.c

Reimplement BarnOwl::add_io_dispatch with AnyEvent

We can emulate the interesting semantics with perl. The one difference
is that perl code can now register an IO dispatch on file descriptors C
code was interested in. This isn't a big deal was Glib can handle
multiple watches on the same FD. Granted, more than one reader on an FD
would cause trouble, but there was nothing stopping perl code from
reading from an FD we cared about anyway.

AnyEvent also does not support select's exceptfd, so this is a slight
behavior change, but probably an uninteresting one.