Prefix all global variables to aid debugging.

Prefix all global variables to aid debugging.

* Also move all options which are set in darkstat.c to a new
  header opt.h and prefix options with opt_.
* Declare all opt_* stuff in darkstat.c.
* Prefix globals from acct.c with acct_.
* Prefix globals from cap.c with cap_.
* Prefix globals from hosts_db.c with hosts_db_.
* Prefix globals from http.c with http_.