MINOR: config: add predicates "version_atleast" and "version_before" to cond blocks

MINOR: config: add predicates "version_atleast" and "version_before" to cond blocks

These predicates respectively verify that the current version is at least
a given version or is before a specific one. The syntax is exactly the one
reported by "haproxy -v", though each component is optional, so both "1.5"
and "2.4-dev18-88910-48" are supported. Missing components equal zero, and
"dev" is below "pre" or "rc", which are both inferior to no such mention
(i.e. they are negative). Thus "2.4-dev18" is older than "2.4-rc1" which
is older than "2.4".