Changing the default shell to /bin/sh brought up a long-standing bug on OS X,
that /bin/sh re-exec's itself to /bin/bash, so it needs one more resume when you
are using it as the shell than /bin/bash did or you will stop at the start of your
program, rather than running it.

So I added a Platform API to get the number of resumes needed when launching with
a particular shell, and set the right values for Mac OS X.

<rdar://problem/14935282>