BUG/MINOR: proxy: pass PR_FL_DEF_EXPLICIT_MODE through cascaded defaults

BUG/MINOR: proxy: pass PR_FL_DEF_EXPLICIT_MODE through cascaded defaults

Basha Mougamadou reported that in the following setup:

   defaults http
      mode http
      timeout client 10s
      timeout server 10s

   defaults fast-http from http
      timeout server 1s

Trying to add a backend using "from fast-http" would be rejected because
the "fast-http" section has no explicit "mode" directive. This is indeed
true, but not correct in that it is inherited from the "http" section
where it is explicit, so there it must be considered as explicit as well.
The reason is that the flag PR_FL_DEF_EXPLICIT_MODE is not inherited from
defaults sections in proxy_defproxy_cpy(). Let's just add it to the set
of such inherited flags to fix the problem.

This must be backported to 3.4.