handle execve of a #! script

For late injection I would expect things to mostly work. However, this is
not something that we have spent any time on, and we have no tests of it,
so there may well be something that is not quite right such as when looking
up child config vars or when trying to handle cross-arch (i.e., 32-bit app
execs 64-bit child) due to failing to get the proper final ELF binary.

For early injection, however, I would expect DR to actually need to parse
the magic comment for correct execution of the proper binary, b/c it passes
the app path to itself on the other side. This will be a pain b/c it can
recurse up to 4 times through other scripts.