Sanitise environment before calling out to system shell

On OS X Yosemite, the system sets the environment variable `$PATH` multiple times.  This causes undefined behaviour in software that uses the `$PATH` environment variable, because the POSIX specification does not specify which value should be used.

In particular, it was found that `put ":/usr/local/bin" after $PATH; put shell("env")` showed that the LiveCode was modifying the first occurrence of $PATH in the environment, but `bash` was using the last.

To address this, use execle(3) rather than execl(3) and provide an explicit environment variable array using the environment values from LiveCode's global variables.
I'd suggest just adding an accessor to the MCVariable class here 'isenv()' or some such - rather than checking like this