MINOR: proxy: monitor-uri works with tcp->http upgrades

MINOR: proxy: monitor-uri works with tcp->http upgrades

Currently, we have a check in proxy_cfg_ensure_no_http() that generates a
warning if the monitor-uri is configured on a proxy that doesn't have
mode HTTP enabled.

However, when we give a look at monitor-uri implementation, it's not
100% correct. Indeed, despite the warning message, the directive will
still be evaluated when HTTP upgrade occurs from a TCP frontend.
Thus the error is misleading.

To make the error message comply with the actual behavior, the check was
moved alongside other checks that accept both native HTTP mode or HTTP
upgrades in cfgparse.c.