Build a subgraph in CFG for each function

[x87]

• a script file consists of one or more functions (main + each gosub)
• each function has its own subgraph in CFG
• isHeaderBlock property in basicBlocks is true for the first BB in a subgraph

[thelink2012]

Something to notice:

• There are instances of GOTOs into GOSUB labels in a certain mission script in III I think.
• START_NEW_SCRIPT and LAUNCH_MISSION should fit this nicely.