Support for (system '("echo" "foo" "bar"))

[Rob Browning]

I've just finished adding support for calling (system cmd) with a list
argument.  This causes it to use fork/execlp/waitpid nstead of calling
underlying OS's system() function.  This should be safer in cases
where it's acceptable because it avoids /bin/sh interpretation and the
attendant risks like sneaky IFS settings, unexpected expansions, etc.

This idea was borrowed from perl, and seems fairly useful, but I
wanted to see if there were any substantial objections before I
commit.  I suppose if this addition is acceptable, I should probably
also see about doing something similar to open-*-pipe.

Also, if people don't like this approach, perhaps there's some
alternate (and more suitable) way to accomplish something similar,
i.e. making it easy to more safely execute a sub-process when you
don't need shell interpolation.

-- 
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
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