1.0.46.19: add :NOT-NULL option to C-STRING type

1.0.46.19: add :NOT-NULL option to C-STRING type

 By default NIL is a valid C-STRING, translated to and from C's NULL. This is
 very convenient because many C functions that normally return strings return
 NULL to indicate "false" or "don't know" -- and less commonly also special
 case it as an argument.

 There are however many C functions that don't check for NULL, so we want a
 non-horrible way to say that NIL is not a good value to pass on...

 ...it remains to be seen if this is non-horrible enough, but at least it
 fixes a bunch of memory faults from doing things like (posix-getenv nil), and
 replaces them with type-errors.

 Not all C-STRING types have been audited yet, just a bunch of the more
 obvious ones.