Changed the way we handle entities in message/rfc822 types. $msg->entity_id++

Changed the way we handle entities in message/rfc822 types. $msg->entity_id++
would increment by 0.1 in php 4.0.4 but would increment by 1.0 in php 4.0.6.
This meant that a message/rfc822 type with only a text/plain part would label
the text/plain part entity 3.0 instead of 2.1.  This should fix that problem.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@3469 7612ce4b-ef26-0410-bec9-ea0150e637f0