Add cURL support to HTTP Client

- Add HTTP_PROXY_EXCLUDE option when cURL is used
- Show HTTP client backend in about page
- Fallback to legacy Stream Contexts if cURL extension is not available

Dockerfile

@@ -12,7 +12,7 @@ RUN apk update && \
     apk add openssl unzip nginx bash ca-certificates s6 curl ssmtp mailx php7 php7-phar php7-curl \
     php7-fpm php7-json php7-zlib php7-xml php7-dom php7-ctype php7-opcache php7-zip php7-iconv \
     php7-pdo php7-pdo_mysql php7-pdo_sqlite php7-pdo_pgsql php7-mbstring php7-session php7-bcmath \
-    php7-gd php7-mcrypt php7-openssl php7-sockets php7-posix php7-ldap php7-simplexml && \
+    php7-gd php7-mcrypt php7-openssl php7-sockets php7-posix php7-ldap php7-simplexml php7-curl && \
     rm -rf /var/cache/apk/* && \
     rm -rf /var/www/localhost && \
     rm -f /etc/php7/php-fpm.d/www.conf