Relax the restriction that the hash table key must be a string
by allowing an arbitrary hash code generator + comparison func
to be provided

* util/hash.c, util/hash.h: Allow any pointer as a key
* internal.h: Include stdbool.h as standard.
* conf/domain_conf.c, conf/domain_conf.c,
  conf/nwfilter_params.c, nwfilter/nwfilter_gentech_driver.c,
  nwfilter/nwfilter_gentech_driver.h, nwfilter/nwfilter_learnipaddr.c,
  qemu/qemu_command.c, qemu/qemu_driver.c,
  qemu/qemu_process.c, uml/uml_driver.c,
  xen/xm_internal.c: s/char */void */ in hash callbacks