lib: combine two YANG schema iteration functions into one

lib: combine two YANG schema iteration functions into one

Combine yang_snodes_iterate_module() and yang_snodes_iterate_all()
into an unified yang_snodes_iterate() function, where the first
"module" parameter is optional. There's no point in having two
separate YANG schema iteration functions anymore now that they are
too similar.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>