boilerplate: new G2PEntry handling successors

The former G2PEntry, which was in fact an entry point to the main
function, is now renamed G2PMain.

G2PEntry now handles goroutines, and adds their successors to the
main function's successor.
1 job for am/goroutines in 37 seconds
latest
Status Job ID Name Coverage
  Test
passed #66585
debian-stable-gcc

00:00:37