MEDIUM: proxy: Reject some header names for 'http-send-name-header' directive

MEDIUM: proxy: Reject some header names for 'http-send-name-header' directive

From time to time, we saw the 'http-send-name-header' directive used to
overwrite the Host header to workaround limitations of a buggy application.
Most of time, this led to troubles. This was never officially supported and
each time we strongly discouraged anyone to do so. We already thought to
deprecate this directive, but it seems to be still used by few people. So
for now, we decided to strengthen the tests performed on it.

The header name is now checked during the configuration parsing to forbid
some risky names. 'Host', 'Content-Length', 'Transfer-Encoding' and
'Connection' header names are now rejected. But more headers could be added
in future.