MINOR: checks: update dynamic environment variables in external checks

commit 9ede66b0 introduced an environment variable (HAPROXY_SERVER_CURCONN) that
was supposed to be dynamically updated, but it was set only once, during its
initialization.

Most of the code provided in this previous patch has been rewritten in order to
easily update the environment variables without reallocating memory during each
check.

Now, HAPROXY_SERVER_CURCONN will contain the current number of connections on
the server at the time of the check.
DOC: checks: environment variables used by "external-check command"

Add some documentation about the environment variables available with
"external-check command". Currently, only one of them is dynamically updated
on each check : HAPROXY_SERVER_CURCONN.