cURL does not honour default_socket_timeout

SabreDAV uses cURL for the requests and as this does not honour
default_socket_timeout, setting it is useless and confusing as
people will expect the request to timeout faster than it actually
will do.

One has to use
 curl_setopt($curl, CURLOPT_TIMEOUT, x)
or
 curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, x)
but there is currently no way to make SabreDAV pass this to cURL.
Merge pull request #3563 from evgeni/curl-no-timeout

cURL does not honour default_socket_timeout