Fix "jsub -continuous" quoting issues.

"jsub -continuous" failed if either an argument to an option
or a part of the command line to be executed contained "'"s.
This fixes bug 48334 by using 3-argument open for the former
and String::ShellQuote for the latter.

Bug:48334
Change-Id: I5198d2ee0592f8756905a18139ad2f50fa1cba84