Add all valid flags for htmlspecialchars() to php-constants

The function htmlspecialchars() accepts a number of constants as
flags.  We properly highlight ENT_COMPAT and ENT_NOQUOTES because
those are part of the php-constants list.  But we are missing all of
the rest, like ENT_QUOTES.  This patch adds every flag given in the
documentation for htmlspecialchars():

    http://php.net/manual/en/function.htmlspecialchars.php

Github-issue: 48
Update php-mode-modified for the new year