[fortran, patch] Implement EXECUTE_COMMAND_LINE

[FX]

This is a respin of my patch last year (http://gcc.gnu.org/ml/fortran/2009-05/msg00044.html), with some more testing, a testcase and documentation.

Answering to Steve's review of last year:
  * the asynchronous path is really asynchronous, because system() is called from the child process (which we never wait for)
  * I noted in the doc that, because of the implementation, the child might ignore/block signals

Bootstrapped and regtesting on x86_64-darwin, OK to commit?

FX


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