utils: Reimplement ll_idx_n2a() and introduce ll_idx_a2n()

utils: Reimplement ll_idx_n2a() and introduce ll_idx_a2n()

Now all users of ll_idx_n2a() replaced with ll_index_to_name() we can
move it's functionality to ll_index_to_name() and implement index to
name conversion using snprintf() and "if%u".

Use %u specifier in "if%..." template consistently: network device
indexes are always greather than zero.

Also introduce ll_idx_n2a() conterpart: ll_idx_a2n() that is used
to translate name of the "if%u" form to index using sscanf().

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>