Add a "User-Agent" line to our HTTP proxy requests.
Add a "User-Agent" line to our HTTP proxy requests.

Apparently some proxy servers can be configured to deny requests based
on User-Agent, even for CONNECT connections. See
https://bugs.kde.org/show_bug.cgi?id=155707#c155 for an example (packet
dump in #c157, analysis in #c158).

So send a User-Agent header with value "Mozilla/5.0", hoping that this
will be enough to allow the connection. I hope it will, because other
tools like libtool send something completely different:

User-Agent: curl/7.19.5 (i586-mandriva-linux-gnu) libcurl/7.19.5
GnuTLS/2.8.1 zlib/1.2.3 c-ares/1.6.0 libidn/1.15 libssh2/1.0

Reviewed-by: TrustMe

RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,radekp/qt,KDE/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,RLovelett/qt,RLovelett/qt,RLovelett/qt,RLovelett/qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,KDE/qt,radekp/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,radekp/qt,radekp/qt,radekp/qt,KDE/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,KDE/qt,igor-sfdc/qt-wk,RLovelett/qt,pruiz/wkhtmltopdf-qt,radekp/qt,igor-sfdc/qt-wk,RLovelett/qt,RLovelett/qt,radekp/qt,KDE/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,KDE/qt,KDE/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk