Add vararg variants for error/log API calls

Add API calls:

- duk_error_va()
- duk_push_error_object_va()
- duk_log_va()

Also fix return value issue for duk_push_error_object() when variadic macros
are not available (i.e. legacy environments).  The comma expression used by
the macro wasn't parenthesized which doesn't work correctly when the return
value is assigned to a variable.
API tests for new vararg calls
API docs for new vararg API call variants
Code issues note for vararg comma expr parens
Release note: vararg variants
Merge branch 'api-add-valist-calls'

Add vararg variants: duk_error_va(), duk_push_error_object_va(), duk_log_va()
(see GH-65).

Also fix duk_push_error_object() problem when variadic macros are not
available (i.e. legacy platforms).