This adds a wrapper (uci_load_validate) for uci_validate_section() that
allows callers (through a callback function) to access the values set by
uci_validate_section(), without having to manually declare a
(potentially long) list of local variables.
The callback function receives two arguments when called, the config
section name and the return value of uci_validate_section().
If no callback function is given, then the wrapper exits with the value
returned by uci_validate_section().
This also updates several init scripts to use the new wrapper function.
Signed-off-by: Jeffery To <jeffery.to@gmail.com>