Better solution to the execve() issue on the Mac.  Flag when we're in
an exec function and skip reporting any other calls.  Also implements
call_execvp() since the call does not have execvpe().
Only clear the in_exec flag if we set it.  Otherwise the first failure
in say execve() from an execvp() causes execve() to start printing.
Also fix typo in call_execvpe() from last commit, where I used "execlp"
instead of "execvp".
Remove a debug print I left in the test on accident.