net: provide alternatives to {ntoh, hton}[sl] funtions with cleaner semantics ...

ntohl always converts 32 bits even on archs where sizeof(long) == 8.
"ntoh32" is a much more intuitive name here. Also the name of the 64 bit
variant that is also added isn't questionable.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>