Problem:
Assigning to the local value of a global-local boolean option
('autoread', 'autocomplete', 'fsync') aborts:
Assertion failed: (curval.type == newval.type), function
ex_let_option, file vars.c, line 1408.
ex_let_one
ex_let_vars
ex_let
execute_cmd0
do_cmdline
call_user_func
...
eval_map_expr
vgetorpeek
vgetc
state_enter
main
A global-local option may have local value `kObjectTypeUnset`, but
`ex_let_option()` guards only `kObjectTypeNil`.
Solution:
When curval is Unset, resolve it to the inherited global value.
ceccca7780