BUG/MINOR: server: alloc dynamic srv ssl ctx if proxy uses ssl chk rule

BUG/MINOR: server: alloc dynamic srv ssl ctx if proxy uses ssl chk rule

The ssl context is not initialized for a dynamic server, even if there
is a tcpcheck rule which uses ssl on the related backed. This will cause
the check initialization to failed with the message :
  "Out of memory when initializing an SSL connection"

This can be reproduced by having the following config in the backend :
  option tcp-check
  tcp-check connect ssl
and create a dynamic server with check activated and a ca-file.

Fix this by calling the prepare_srv xprt callback when the proxy options
PR_O_TCPCKH_SSL is set.

Check support for dynamic servers has been merged in the current branch.
No backport needed.