Support JavaScript property name expansion using JS:<name> @-code where name is

the name of a scalar property in either the current scope (by default) or the
scope of the object passed to:
- bbs.menu()
- console.putmsg()
- console.printfile()
- console.printtail()

The 'name' cannot be an array element (e.g. myprop[0]) or a nested object
reference (e.g. myobj.myprop): just a single property name that can be
converted to a string.

Also, bbs.menu() now accepts an optional print-mode argument (default: P_NONE).