Change/Feature request : suppress CON code warnings about official GRP's CON file issues

Atomic version examples:

• the two "'else' with no 'if'" warnings in the GAME.CON
• two already defined sounds in the GAME.CON
• missing VOC files in USER.CON

Instead: Show only one message (FOOBAR.CON's checksum is known):

"Note: suppressing warnings about legacy CON code issues in well-known FOOBAR.CON"