MINOR: proxy: Review options flags used to configure healthchecks

MINOR: proxy: Review options flags used to configure healthchecks

When healthchecks were configured for a proxy, an enum-like was used to
sepcify the check's type. The idea was to reserve some values for futur
types of healthcheck. But it is overkill. I doubt we will ever have
something else than tcp and external checks. So corresponding PR_O2 flags
were slightly reviewed and a hole was filled.

Thanks to this change, some bits were released in options2 bitfield.