LP#1436047: Allow disabling of "bang commands" in srfsh

LP#1436047: Allow disabling of "bang commands" in srfsh

srfsh has the ability to execute commands via system() calls using
the common "!command" syntax.  This is very useful, but it would
be nice to be able to turn that functionality off in some cases.

This branch adds argument parsing to detect a new '--safe' command
line parameter, which disables the "!command" syntax.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>