i#1013: add dr_inject_wait_for_process() to drinjectlib

This moves the complexity of waiting for a child process with a timeout up into
drinjectlib so that we don't need to repeat it in every tool frontend.

Fixes issue 1013.

Review URL: https://codereview.appspot.com/7349043

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1916 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6