DEBUG: support reading set-dumpable from HAPROXY_FORCE_DUMPABLE

DEBUG: support reading set-dumpable from HAPROXY_FORCE_DUMPABLE

When debugging regtests, it is super complicated to get a core to be
dumped. It often requires to place a set-dumpable directive in the
config, but when it's generated it's really not convenient. A better
approach in this case is to read the value from an environment variable.

That's what this patch does, by accepting to preset the set-dumpable
options from the contents of HAPROXY_FORCE_DUMPABLE. It's a bit more
relaxed, though, as it accepts "off" to stay disabled, "libs" to just
load the libs, and anything else to enable.

The config and management docs were updated. This could be backported
to older versions if found helpful.