BUG/MEDIUM: check: Skip tcpcheck post-config for external checks

BUG/MEDIUM: check: Skip tcpcheck post-config for external checks

When an external check was configured on a backend, the tcpcheck post config
for backend's servers was still performed instead to be skipped. The led to
a NULL-deref on the tcpcheck ruleset pointer and so to a segfault.

It seems to be only an issue for the 3.4 and higher. However, for older
versions, the tcpcheck post-config is still performed for external checks
and it is not really clean. This can hide some bugs.

For the 3.4, a workaround consists in configuring the backend to use a
tcp-check before configuring the external check:

  backend be
    option tcp-check
    option external-check
    ...

This patch should fix the issue #3407. It could be good to backport it to
all supported versions.