Exec: add option for ruby 1.9 FD behavior for 2.0

Ruby 2.0 changed the behavior of file descriptors when doing a Kernel.exec.
Where in 1.9 it wouldn't automatically close non-standard file descriptors
when doing a Kernel.exec, in 2.0 the default is to close them.

Issue #2628
add some documentation, warn if passing --keep-file-descriptors on < 2.0
don't require what we don't use
Merge pull request #2629 from steved555/master

Exec: add option for ruby 1.9 FD behavior for 2.0
exec flags must go before the command
reword exec docs for clarity