MINOR: checks: Support custom functions to eval a tcp-check expect rules

MINOR: checks: Support custom functions to eval a tcp-check expect rules

It is now possible to set a custom function to evaluate a tcp-check expect
rule. It is an internal and not documentd option because the right pointer of
function must be set and it is not possible to express it in the
configuration. It will be used to convert some protocol healthchecks to
tcp-checks.

Custom functions must have the following signature:

  enum tcpcheck_eval_ret (*custom)(struct check *, struct tcpcheck_rule *, int);