sim: use socklen_t with accept()

sim: use socklen_t with accept()

The accept() function takes a socklen_t, not an int.  Using an int causes:
dv-sockser.c: In function 'connected_p':
dv-sockser.c:273: warning: pointer targets in passing argument 3
                           of 'accept' differ in signedness

So use the same socklen_t detection code as gdb and convert the accept().