Add a "strict" mode

[fingolfin]

Let's add a strict mode (like e.g. perl has) which warns resp. errors on extra things:

• if a function returns sometimes an obj and sometimes nothing (see PR #466)
• invalid backslash escaped chars inside strings (see PR #619)
• syntax errors are actual errors (see issue #1981)
• warn if a function has unused locals (i.e. local variables that are never read from; so completely unused, or only ever assigned to)
• ...

This mode could be enabled or disabled via a command line switch.

[fingolfin]

So to elaborate a bit, here is what I'd do:

• add a global boolean flag to the kernel, say StrictMode, which defaults to 0 / false.
• add a command line option --strict which changes the default to 1 / true (can be added in src/system.c next to the other command line options)
• then look at the code in the two pull requests referenced above; try to adapt their patches so that their warnings are only printed if StrictMode is 1 / true.
• think about other things it could do, and/or ask about
• also consider exposing the state of the mode to the GAP language level somehow (say by setting a global variable)