Add Breaks agains php7.0-curl, php7.2-sodium and gforge-common.

 * php7.3-common: Add Breaks against php7.0-curl for smoother upgrades from
   stretch.  (Closes: #925106)
 * php7.3-common: Add Breaks against gforge-common from jessie which uses a
   deprecated constructor syntax.

debian/php-common.substvars.extra

-php-common:Breaks=php7.2-sodium
+php-common:Breaks=php7.0-curl (<< 7.0.33-1~), php7.2-sodium (<< 7.2.12~), gforge-common (<< 6)