BUG/MINOR: quic: newreno QUIC congestion control algorithm no more available

BUG/MINOR: quic: newreno QUIC congestion control algorithm no more available

There is a typo in the statement to initialize this variable when selecting
newreno as cc algo:
      const char *newreno = "newrno";
This would have happened if #defines had be used in place of several const char *
variables harcoded values.

Take the opportunity of this patch to use #defines for all the available cc algorithms.

Must be backported to 2.9.

(cherry picked from commit b9a163e7e1ded0dc9cda78a9da652f14f424605a)
Signed-off-by: Willy Tarreau <w@1wt.eu>