[MEDIUM] checks: group health checks methods by values and save option bits

[MEDIUM] checks: group health checks methods by values and save option bits

Adding health checks has become a real pain, with cross-references to all
checks everywhere because they're all a single bit. Since they're all
exclusive, let's change this to have a check number only. We reserve 4
bits allowing up to 16 checks (15+tcp), only 7 of which are currently
used. The code has shrunk by almost 1kB and we saved a few option bits.

The "dispatch" option has been moved to px->options, making a few tests
a bit cleaner.