Added support for set [{+|-}{e|x|xe|ex}] [<name> <value>]

Set the 'exit on error control' and/or 'print a trace' of commands when parsing scripts in NSH.  The settinngs are in effect from the point of exection, until they are changed again, or in the case of the init script, the settings are returned to the default settings when it exits.

Included child scripts will run with the parents settings and changes made in the child script will effect the parent on return.

Use 'set -e' to enable and 'set +e' to disable (ignore) the exit condition on commands. The default is -e. Errors cause script to exit.

Use 'set -x' to enable and 'set +x' to disable (silence) printing a trace of the script commands as they are ececuted. The default is +x. No printing of a trace of script commands as they are executed.