add a new pre-syscall app2app event to truly skip such that no other component sees the syscall

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 19, 2013 11:30:31_

Split from issue #424 where we decided that most tool uses of skipping a
syscall really want the app to think the syscall happened, and that thus
other tool components should see the syscall event.

There may be tools that do want to completely remove the syscall to the
effect of the app having never executed it in the first place. With
vsyscall or other cases of not knowing the syscall # statically this can be
awkward to do from a bb event. We could provide a separate "app2app"-like
pre-syscall event. But unless we have a compelling use case this will
remain a low-priority nice-to-have.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1324_