Global JS function parameter validation and updated return types

Many global functions now throw an exception when null or undefined is passed
as a required parameter where null or undefined makes no sense.
e.g. "Argument #1 is an unexpected 'null' or 'undefined' value"
Example exception: is-a/has-a type functions will still return false when
passed null or undefined.

Changed global function return types:
- write() now always returns undefined
- printf() always returns a string, never undefined or garbage string
- prompt() returns null (not undefined) upon no input (as documented)

sbbs works more like jsexec:
- read(0) now returns an empty string instead of undefined
- readln(0) no longer waits for enter

jsexec works more like sbbs:
- add missing write_raw() global function, alias for write()

Include expected number of argment in "Insufficient Arguments" exception
message.

Update JSDOCS for some global methods:
- file_chmod returns boolean, not number
- flags_str requires a minimum of one argument

New tests to validate expected exceptions and return types based on usage.