Remove duplicate param escaping code

Remove duplicate param escaping code

wfEscapeShellArg() can handle multiple params, escaping each.
This patch changes wfShellExec() to call wfEscapeShellArg() directly
instead of doing the gluing itself.

This patch also extends wfEscapeShellArg() to accept an array parameter
optionally instead of as separate args, which is often useful.

Added also unit test cases for single, multiple args, and single array args.

Change-Id: I7a0761cc2ba98c210a9eacadd12da407d933e42a