prog: better call-to-call priority calculation

[dvyukov]

The call priorities calculation is quite basic:
https://github.com/google/syzkaller/blob/master/prog/prio.go
Some things to improve:

• Static priorities for call pairs should account for resource direction. For example, open produces an fd, write uses an fd, so it makes sense to add write after open, but not the other way around.
• Dynamic priorities for call pairs should account for before/after. For example, setsockopt is only interesting after socket, but not the other way around.
• The weights for resource usage may need tuning.
• When we insert a call, we choose a "base" call to use in priority selection of the new call:
  

  syzkaller/prog/rand.go

  Line 473 in 040fda5

  call = p.Calls[r.Intn(len(p.Calls))].Meta.ID

  . If we insert a call into the middle of the program during mutation, we should choose the "base" call only from the call before the insertion program (not all calls in the program).

[dvyukov]

Moved out of #534