BUG/MINOR: check: ensure check-reuse is compatible with SSL

BUG/MINOR: check: ensure check-reuse is compatible with SSL

SSL may be activated implicitely if a server relies on SSL, even without
check-ssl keyword. This is performed by init_srv_check() function. The
main operation is to change xprt layer for check to SSL.

Prior to this patch, <use_ssl> check member was also set, despite not
strictly necessary. This has a negative side-effect of rendering
check-reuse-pool ineffective. Indeed, reuse on check is only performed
if no specific check configuration has been specified (see
tcpcheck_use_nondefault_connect()).

This patch fixes check reuse with SSL : <use_ssl> is not set in case SSL
is inherited implicitely from server configuration. Thus, <use_ssl> is
now only set if an explicit check-ssl keyword is set, which disables
connection reuse for check.

This must be backported up to 3.2.