Enforce two illegal import statements that were outlawed in the

reference manual but not checked: Names bound by import statemants may
not occur in global statements in the same scope. The from ... import *
form may only occur in a module scope.

I guess these changes could break code, but the reference manual
warned about them.

Several other small changes

If a variable is declared global in the nearest enclosing scope of a
free variable, then treat it is a global in the nested scope too.

Get rid of com_mangle and symtable_mangle functions and call mangle
directly.

If errors occur during symtable table creation, return -1 from
symtable_build().

Do not increment st_errors in assignment to lambda, because exception
is not set.

Add extra argument to symtable_assign(); the argument, flag, is ORed
with DEF_LOCAL for each symtable_add_def() call.