MINOR: cfgcond: Implement enabled condition expression

MINOR: cfgcond: Implement enabled condition expression

Implement a way to test if some options are enabled at run-time. For now,
following options may be detected:

  POLL, EPOLL, KQUEUE, EVPORTS, SPLICE, GETADDRINFO, REUSEPORT,
  FAST-FORWARD, SERVER-SSL-VERIFY-NONE

These options are those that can be disabled on the command line. This way
it is possible, from a reg-test for instance, to know if a feature is
supported or not :

  feature cmd "$HAPROXY_PROGRAM -cc '!(globa.tune & GTUNE_NO_FAST_FWD)'"