Failures clearly returned from a (SOCKS) proxy now causes this return
code. Previously the situation was not very clear as what would be
returned and when.
In addition: when this error code is returned, an application can use
CURLINFO_PROXY_ERROR to query libcurl for the detailed error, which then
returns a value from the new 'CURLproxycode' enum.
Closes#5770
The purpose with this variable is to provide a port number that is
reasonably likely to not have a listener on the local host so that tests
can try connect failures against it. It uses port 47 - "reserved"
according to IANA.
Updated six tests to use it instead of the previous different ports.
Assisted-by: Emil Engler
Closes#5270
