base-files: fix UCI config parsing and callback handling

There are several long-standing issues present in the UCI shell API as
documented in https://wiki.openwrt.org/doc/devel/config-scripting. They
relate both to high-level, user-defined callback functions used to
process UCI config files, and also to low-level functions used within
scripts generally.

The related problems have been encountered now and in the past, e.g.
https://forum.openwrt.org/viewtopic.php?id=54295

, and include:

a) UCI parsing option() function and user-defined option_cb() callbacks
being erroneously called during processing of "list" config file entries;

b) normal usage of the low-level config_set() unexpectedy calling any
defined option_cb() if present; and

c) handling of the list_cb() not respecting the NO_CALLBACK variable.

Root causes include a function stack "inversion", where the low-level
config_set() function incorrectly calls the high-level option() function,
intended only for processing the "option" keyword of UCI config files.

This change addresses the inversion and other issues, making the option
handling code more consistent and smaller, and simplifying developers'
usage of UCI callbacks.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]