procd: Add wrapper for uci_validate_section()

procd: Add wrapper for uci_validate_section()

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>