Bug fixes for CVA_LIST PTR types

[jayrm]

A few fixes for issues mentioned on freebasic.net forums:
See variadic functions and argument lists in fbc

• disable suspicious warnings on NULL pointer checks for pointers to cva_list types
• disable -gen gcc warning of comparison of distinct pointer types lacks a cast when comparing cva_list pointer types
• disallow ANY ALIAS "modifier" unless it is also a pointer type, like ANY PTR. Allocated data types must have non-zero size.